Switching power supply apparatus

ABSTRACT

A switching power supply apparatus operates with less power consumption as a whole as a result of reduced power loss suffered while the switching operation of the main switching device is being stopped in burst switching control. Burst switching control is achieved by a signal level checker circuit  15  repeatedly turning on and off a switch circuit  17  provided in the line by way of which a switching controller circuit  19  is supplied with operating power. In burst switching control, when the switching operation of a main switching device  5  is being stopped, the supply of operating power to the switching controller circuit  19  is also stopped. This helps reduce the power loss suffered while the switching operation is being stopped, and thus helps reduce the power consumption of the apparatus as a whole.

This nonprovisional application claims priority under 35 U.S.C. § 119(a)on Patent Application No.2002-249260 filed in JAPAN on Aug. 28, 2002,which is herein incorporated by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a switching power supply apparatus usedas a direct-current power source in an electronic appliance.

2. Description of the Prior Art

A conventionally known example of such a switching power supplyapparatus is disclosed, for example, in Japanese Patent ApplicationLaid-Open No. H10-304658. The switching power supply apparatus disclosedin this publication is provided with a main switch that turns on and offa direct current applied to the primary coil of a transformer, asecondary-side rectifying/smoothing circuit that rectifies and smoothesthe on/off signal induced in the secondary coil of the transformer so asto supply it as a main output signal, a subsidiary power source thatrectifies and smoothes the on/off signal induced in the bias coil of thetransformer so as to supply it as a subsidiary supply voltage, an erroramplifier that generates an error voltage signal that represents thedifference between the subsidiary supply voltage output from thesubsidiary power source and a reference voltage, and a comparator thatfeeds the main switch with an on/off control signal in such a way as toreduce the error voltage signal output from the error amplifier. Thisswitching power supply apparatus is further provided with a light-loadswitching controller portion that temporarily stops the on/off operationof the main switch when the main output voltage becomes higher than anupper limit voltage and that restarts the on/off operation of the mainswitch when the main output voltage becomes lower than a lower limitvoltage.

In this conventional switching power supply apparatus, control is soperformed that the on/off operation of the main switch is temporarilystopped when the main output voltage output from the secondary-siderectifying/smoothing circuit becomes higher than the upper limitvoltage, and that the on/off operation of the main switch is restartedwhen the main output voltage becomes lower than the lower limit voltage.

Here, however, while the on/off operation of the main switch istemporarily stopped when the main output voltage becomes higher than theupper limit voltage, operating power is kept supplied to the individualcircuits and control devices provided in the control circuit that drivesand controls the main switch. This causes wasteful power loss.Specifically, in a configuration where the on/off operation of the mainswitch is temporarily stopped when the main output voltage output fromthe secondary-side rectifying/smoothing circuit becomes higher than theupper limit voltage and the on/off operation of the main switch isrestarted when the main output voltage becomes lower than the lowerlimit voltage, i.e., in so-called burst switching control, the operatingpower is kept supplied to all the circuits and control devices providedin the control circuit even while the switching operation is beingstopped. This causes wasteful consumption of the supply current,resulting in wasteful power loss.

Also in the conventional switching power supply apparatuses disclosed inJapanese Patent Applications Laid-Open Nos. 2001-346378 and 2002-58238,as in the switching power supply apparatus described above, even duringthe period in which the switching operation of the main switch is beingstopped in burst switching control, the operating power is kept suppliedto all the circuits and control devices provided in the switching signalcontrolling circuit. This causes wasteful consumption of the supplycurrent, resulting in wasteful power loss.

Incidentally, in the conventional switching power supply apparatusdisclosed in Japanese Patent Application Laid-Open No. 2001-86745, toreduce power consumption in a stand-by state, the switching operation ofthe main switching device is stopped in that state. Thus, the aim ofthis invention is not to reduce the power loss suffered while theswitching operation of the main switching device is being stopped inburst switching control.

SUMMARY OF THE INVENTION

To facilitate a complete understanding of the invention, a glossary ofterms and acronyms is provided on the last page of the specificationbefore the claims.

An object of the present invention is to provide a switching powersupply apparatus that operates with less power consumption as a whole asa result of reduced power loss suffered while the switching operation ofthe main switching device is being stopped in burst switching control.

To achieve the above object, according to one aspect of the presentinvention, a switching power supply apparatus has a serial circuit,including the primary coil of a transformer and a main switching device,connected between a positive and a negative power supply line connectedto a direct-current power source. The switching power supply apparatusoutputs a direct-current voltage obtained by rectifying with a rectifiera high-frequency voltage induced in the secondary coil of thetransformer by the main switching device performing switching operation.Here, the switching power supply apparatus uses as a feedback signal theresult of comparison between the direct-current voltage and apredetermined reference voltage, and drives the main switching device byturning on and off, according to the signal level of the feedbacksignal, supply of operating power to a main switching device drivingsystem that drives the main switching device.

In this switching power supply apparatus according to the invention, forexample, in heavy-load operation, the output voltage decreases. Tocorrect this, a lower-level feedback signal is generated. This causesthe operating power to the main switching device driving system to bekept supplied thereto, and thus the main switching device continuesswitching operation. On the other hand, in light-load operation, whenthe output voltage becomes higher than a predetermined value, ahigher-level feedback signal is generated. This causes the supply of theoperating power to the switching device driving system to be stopped,and thus the main switching device stops switching operation. As aresult, the output voltage returns to the predetermined value.

That is, with this switching power supply apparatus according to theinvention, while the switching operation of the main switching device isbeing stopped in burst switching control, the supply of the operatingpower to the main switching device driving system is also stopped. Thishelps reduce the power loss suffered while the switching operation isbeing stopped, and thus helps reduce the power consumption of theapparatus as a whole.

According to another aspect of the present invention, a switching powersupply apparatus has a serial circuit, including the primary coil of atransformer and a main switching device, connected between a positiveand a negative power supply line connected to a direct-current powersource. The switching power supply apparatus outputs a direct-currentvoltage obtained by rectifying with a rectifier a high-frequency voltageinduced in the secondary coil of the transformer by the main switchingdevice performing switching operation. Here, the switching power supplyapparatus further includes: an output voltage detector that compares thedirect-current voltage obtained through rectification with apredetermined reference voltage and that outputs the result of thecomparison as a feedback signal; a switching controller that drives andcontrols the main switching device according to the feedback signaloutput from the output voltage detector; a signal level checker thatmonitors the signal level of the feedback signal and that outputs anoperation control signal for turning on and off the switching controlleraccording to the monitored signal level; and an operation/nonoperationswitcher that is provided in the line by way of which the switchingcontroller is supplied with operating power and that turns on and offthe switching controller according to the operation control signal fromthe signal level checker.

In this switching power supply apparatus according to the invention, inlight-load operation, when the output voltage tends to increase, i.e.,when the output voltage is higher than a predetermined value, and thusthe signal level of the feedback signal output from the output voltagedetector is, for example, high, the signal level checker feeds theoperation/nonoperation switcher with an operation control signal thatrequests nonoperation, and thus the operation/nonoperation switcherstops the supply of the operating power to the switching controller.

As a result, the main switching device stops switching operation, andthus the output voltage starts to decrease gradually. When the signallevel of the feedback signal from the output voltage detector becomes,for example, low, the signal level checker feeds theoperation/nonoperation switcher with an operation control signal thatrequests operation, and thus the operation/nonoperation switcher startsthe supply of the operating power to the switching controller.

As a result, the main switching device restarts switching operation, andthus the output voltage starts to increase gradually. When the signallevel of the feedback signal becomes high again, the signal levelchecker feeds the operation/nonoperation switcher with an operationcontrol signal that requests nonoperation, and thus theoperation/nonoperation switcher stops the supply of the operating powerto the switching controller. As a result, the main switching devicestops switching operation, and thus the output voltage starts todecrease gradually. As this sequence of operations is repeated, theoutput voltage is kept at the predetermined value.

In this switching power supply apparatus, when the output voltage tendsto decrease, i.e., when the output voltage is lower than a predeterminedvalue, and thus the signal level of the feedback signal output from theoutput voltage detector is, for example, low, the signal level checkerfeeds the operation/nonoperation switcher with an operation controlsignal that requests operation, and thus the operation/nonoperationswitcher continues supplying the operating power to the switchingcontroller so that switching operation is performed continuously.

With this switching power supply apparatus according to the invention,burst switching control is achieved as a result of the signal levelchecker repeatedly turning on and off the operation/nonoperationswitcher provided in the line by way of which the switching controlleris supplied with operating power. Moreover, while the switchingoperation of the main switching device is being stopped in burstswitching control, the supply of the operating power to the switchingcontroller is also stopped. This helps reduce the power loss sufferedwhile the switching operation is being stopped, and thus helps reducethe power consumption of the apparatus as a whole.

Preferably, the feedback signal from the output voltage detector istransmitted to the switching controller through the photodiode of aphotocoupler, and the signal level checker monitors the signal level ofthe feedback signal by comparing the current level flowing through thephototransistor of the photocoupler with a reference current level.

With this configuration, burst switching operation is controlledaccording to the result of comparison between the current value throughthe phototransistor and the reference current value. The signal level ofthe feedback signal (i.e., the current value through thephototransistor) represents the load current value of the switchingpower supply apparatus. Thus, it is possible to correctly set the loadcurrent value at which switching between continuous switching operationand burst switching operation is performed.

In burst switching operation, the output voltage fluctuates. However,since the signal level of the feedback signal represents the outputvoltage value, it is possible to correctly set the upper and lowerlimits of the output voltage.

Preferably, a current detection resistor is connected in series with thephototransistor of the photocoupler, and the signal level checker turnson and off the switching controller by feeding the switching controllerwith, as the operation control signal, a signal obtained by comparingthe voltage drop across the current detection resistor with the voltageof a current level check reference power source.

With this configuration, it is possible to make the signal level checkerdetect the signal level of the feedback signal according to the voltagedrop across the current detection resistor, compare the signal levelwith the voltage of the current level check reference power source, andturn on and off the supply of the operating power to the switchingcontroller according to the result of the comparison.

Preferably, the operating power of the switching controller is suppliedby way of the start-up current supply line by way of which a start-upcurrent is supplied from the positive power supply line through astart-up resistor, or by way of the steady-operation current supply lineby way of which a voltage induced in the subsidiary coil of thetransformer is supplied after being rectified with a serial circuitcomposed of a plurality of diodes, and the operating power of the signallevel checker is supplied from subsidiary control power extracted from anode between the plurality of diodes.

With this configuration, when the switching power supply apparatusstarts to start up, it is possible to prevent, by the action of thediodes, the current that is supposed to flow to the start-up currentsupply line from flowing to the steady-operation current supply line.This helps reduce the time required for start-up, and also helps reducethe resistance of the start-up resistor and thereby reduce powerconsumption.

In other words, as compared with a switching power supply apparatus thatis not provided with the function of controlling burst switching in sucha way as to stop the supply of the operating power to the switchingcontroller that performs burst switching operation when the switchingpower supply apparatus starts to start up, the switching power supplyapparatus of the present invention starts up in as short a time whilereducing the unnecessary power consumption by the start-up resistor.

Preferably, the operating power of the signal level checker and thephototransistor of the photocoupler is supplied from subsidiary controlpower extracted from a node between a plurality of diodes constituting aserial circuit provided in the steady-operation current supply line byway of which a voltage induced in the subsidiary coil of the transformeris supplied after being rectified with the plurality of diodes.

With this configuration, when the switching power supply apparatusstarts to start up, the diodes prevent the start-up current from flowingto the subsidiary control power. This helps shorten the start-up time.Moreover, in the steady operation, the direct-current voltage obtainedby rectifying the voltage induced in the subsidiary coil of thetransformer is fed as the operating power to the signal level checkerand the phototransistor of the photocoupler. This ensures stableoperation.

Preferably, the switching controller is realized as a PWM controlcircuit that outputs, as the drive signal with which to drive the mainswitching device, a pulse signal that is pulse-width-modulated accordingto the voltage level of the feedback signal from the output voltagedetector.

With this configuration, the main switching device is driven with adrive signal accurately commensurate with the voltage level of thefeedback signal. This helps enhance the stability of the output voltageof the switching power supply apparatus.

Preferably, used as the PWM control circuit is a PWM control IC (forexample, an IC with the product number FA5511 manufactured by FujiElectric Co., Ltd.) that is realized as an integrated circuit chiphaving at least an FB terminal to which a voltage related to thefeedback signal is input and a CS terminal to which a voltage forenabling or disabling an internal circuit is input.

With this configuration, it is possible to reduce the space occupied bythe circuit that drives the main switching device, and to enhance thestability of the output voltage, leading to miniaturization of theapparatus.

Preferably, when a PWM control IC is used as the switching controller, astart-up corrector is additionally provided to correct the start-up ofthe PWM control IC; a first resistor is connected between the FBterminal of the PWM control IC and the negative power supply line; thesignal level checker feeds a CS terminal controller, which serves as theoperation/nonoperation switcher, and the FB terminal with the operationcontrol signal and an inverted feedback signal, respectively, accordingto the result of checking of the signal level of the feedback signal;the CS terminal controller connects and disconnects the CS terminal ofthe PWM control IC to and from the negative power supply line accordingto the operation control signal; and the start-up corrector connects anddisconnects, through a second resistor, the FB terminal to and from thenegative power supply line according to the voltage level of thesubsidiary control power.

With this configuration, at the start-up of the switching power supplyapparatus, when the voltage of the subsidiary control power increases,immediately before a current starts to flow through the phototransistor,the start-up corrector connects the second resistor in parallel with thefirst resistor and thereby reduces the resistance between the FBterminal and the negative power supply line. This causes the potentialat the FB terminal to decrease. In this way, when the switching powersupply apparatus starts to start up, the voltage at the FB terminal iskept at the optimum level to permit reliable rising of the outputvoltage. In addition, in the steady state, the switching power supplyapparatus is permitted to output a reliably stabilized voltage.

Preferably, the signal level checker includes a pair of transistorshaving the emitters thereof connected together to form a comparator,with the base of one of the transistors connected to the node betweenthe current detection resistor and the phototransistor, with the base ofthe other of the transistors connected to the current level checkreference power source, with the collector of the one of the transistorsconnected to the FB terminal of the PWM control IC, and with thecollector of the other of the transistors connected to the CS terminalcontroller.

With this configuration, it is possible to easily realize the comparatorfor comparing the signal level of the feedback signal with the currentlevel of the current level check reference power.

Preferably, the CS terminal controller includes an NPN-type transistorhaving the collector thereof connected to the CS terminal of the PWMcontrol IC, having the emitter thereof connected to the negative powersupply line, and having the base thereof connected to the collector ofthe other of the transistors included in the signal level checker.

With this configuration, where the CS terminal controller is providedwith the NPN transistor connected in the manner described above, it ispossible, with a simple configuration, to enable and disable the PWMcontrol IC.

Preferably, the start-up corrector includes: a serial circuit composedof a Zener diode and a plurality of resistors connected between the lineof the subsidiary control power and the negative power supply line; andan NPN-type transistor having the base thereof connected to a nodebetween the resistors, having the collector thereof connected throughthe second resistor to the FB terminal of the PWM control IC, and havingthe emitter thereof connected to the negative supply power line.

With this configuration, where the start-up corrector is provided withthe serial circuit and the NPN-type transistor described above, it ispossible, with a simple configuration, to make the switching powersupply apparatus output a reliably stabilized voltage in the steadyoperation.

Preferably, the signal level checker includes, for generation of thereference voltage, voltage division resistors, of which alower-potential-side resistor is divided into two resistors, with thenode therebetween connected through a diode to the CS terminal of thePWM control IC.

With this configuration, by varying the resistances of the individualdivision resistors for generating the reference voltage, it is possibleto freely and accurately set the fluctuation width and burst switchingperiod of the output voltage in burst switching operation. Inparticular, by making the fluctuation width of the output voltage aswide as applications permit, it is possible to reduce unnecessary powerconsumption in burst switching operation.

Preferably, the switching power supply apparatus further includes: acapacitor connected between the CS terminal of the PWM control IC andthe negative power supply line; and a diode connected between thecapacitor and the CS terminal.

With this configuration, in burst switching operation, it is possible toquicken, by the action of the diode, the fluctuation of the voltagelevel at the CS terminal and thereby quicken the speed of switchingbetween a state in which switching operation is performed and a state inwhich switching operation is stopped. Moreover, in burst switchingoperation, it is possible to reduce the fluctuation width of the outputvoltage and increase the accuracy of the upper and lower limits of theoutput voltage. Moreover, when the load current abruptly increasesduring burst switching operation, it is possible to shorten the timerequired to shift to continuous switching operation and thereby preventa decrease in the output voltage.

Preferably, when a PWM control IC is used as the switching controller,the switching power supply apparatus further includes: a currentadjuster connected between the FB terminal of the PWM control IC and thenegative power supply line to adjust the current output from the FBterminal according to the signal level of the feedback signal; and a CSterminal controller that serves as the operation/nonoperation switcherby connecting and disconnecting the CS terminal of the PWM control IC toand from the negative power supply line according to an output signal ofthe signal level checker.

With this configuration, at the start-up of the switching power supplyapparatus, the current adjuster adjusts the voltage at the FB terminalof the PWM control IC to a high value. Thus, the PWM control IC makesthe main switching device perform switching operation with a greaton-state duty, and thereby reduces the start-up time. Moreover, the CSterminal controller connects and disconnects the CS terminal of the PWMcontrol IC to and from the negative power supply line according to theoutput signal of the signal level checker, and thereby turns on and offthe PWM control IC.

Preferably, the current adjuster includes an NPN-type transistor havingthe collector thereof connected to the FB terminal of the PWM controlIC, having the emitter thereof connected through a resistor to thenegative power supply line, and having the base thereof connected to theline of the feedback signal.

With this configuration, the current adjuster has a simplerconfiguration than the start-up corrector, but nevertheless achieves thesame effect. That is, it is possible, with a simpler configuration, toreduce the start-up time of the switching power supply apparatus.

According to another aspect of the present invention, the switchingpower supply apparatus has a serial circuit, including the primary coilof a transformer and a main switching device, connected between apositive and a negative power supply line connected to a direct-currentpower source. The switching power supply apparatus outputs a desireddirect-current voltage by controlling the main switching deviceaccording to a feedback signal obtained as a result of comparisonbetween a direct-current voltage obtained through rectification of ahigh-frequency voltage induced in the secondary coil of the transformerby the main switching device performing switching operation and apreviously set reference voltage. Here, the signal level of the feedbacksignal is compared with the signal level of a previously generatedoscillation signal. According to the result of the comparison, theon-state duty of the drive signal to be fed to the main switching deviceis determined and switching between burst switching control andcontinuous switching control is performed. Moreover, while the switchingoperation of the main switching device is being stopped in burstswitching control, supply of the operating power for driving the mainswitching device is stopped.

In this switching power supply apparatus according to the invention, theon-state duty of the drive signal to be fed to the main switching deviceis determined according to the result of comparison between the signallevel of the previously generated oscillation signal and the signallevel of the feedback signal. This makes it possible to accuratelycontrol the switching of the main switching device. Moreover, switchingbetween burst switching and continuous switching is also performedaccording to the result of the comparison. This makes it possible toaccurately perform the switching. Moreover, while the switchingoperation of the main switching device is being stopped, the supply ofthe operating power for driving the main switching device is stopped.This helps reduce the power loss suffered while the switching operationis being stopped, and thus helps reduce the power consumption of theapparatus as a whole.

Preferably, burst switching control is achieved by turning on and offthe supply of operating power to the switching controller that drivesthe main switching device. This helps reduce the power loss sufferedwhile the switching operation is being stopped.

Preferably, when a PWM control IC is used as the switching controller, acapacitor is connected between the FB terminal of the PWM control IC andan internal power terminal connected to an internal power supply line.

With this configuration, in a case where, for phase compensation of theoutput voltage stabilizing control system, a serial circuit composed ofa capacitor and a resistor is connected between the FB terminal of thePWM control IC and the negative power supply line, even when the loadcurrent abruptly increases during burst switching operation, it ispossible to quicken the control of the burst switching operation controlsystem as much as possible and thereby prevent a decrease in the outputvoltage of the switching power supply apparatus. In addition, it ispossible to reduce unnecessary power consumption in the burst switchingoperation.

Preferably, when a PWM control IC is used as the switching controller, aserial circuit composed of a capacitor and a resistor is connectedbetween the FB terminal of the PWM control IC and an internal powerterminal connected to an internal power supply line.

With this configuration, in a case where, for phase compensation of theoutput voltage stabilizing control system, a serial circuit composed ofa capacitor and a resistor is connected between the FB terminal of thePWM control IC and the negative power supply line, even when the loadcurrent abruptly increases during burst switching operation, it ispossible to quicken the control of the burst switching operation controlsystem as much as possible and thereby prevent a decrease in the outputvoltage of the switching power supply apparatus. In addition, it ispossible to achieve phase compensation in the output voltage stabilizingcontrol system with almost no effects on the burst switching operationcharacteristics.

Preferably, the current adjuster includes an NPN-type transistor havingthe collector thereof connected to the FB terminal of the PWM controlIC, having the emitter thereof connected through a resistor to thenegative power supply line, and having the base thereof connected to theline of the feedback signal, and in series with the resistor connectedbetween the base of the NPN-type transistor and the negative powersupply line is connected an NPN-type transistor having the collector andbase thereof connected together.

With this configuration, even when characteristics change as temperaturevaries, the current adjuster can suppress the variation of thepredetermined current value (of the load current) at which switchingbetween burst switching operation and continuous switching operation isperformed. This helps stabilize the output voltage.

Preferably, when a PWM control IC is used as the switching controller, astart-up corrector is additionally provided to correct the start-up ofthe PWM control IC; a start-up switcher is additionally provided to turnon and off the supply of operating power to the signal level checker; afirst resistor is connected between the FB terminal of the PWM controlIC and the negative power supply line; the signal level checker feeds aCS terminal controller, which serves as the operation/nonoperationswitcher, and the FB terminal with the operation control signal and aninverted feedback signal, respectively, according to the result ofchecking of the signal level of the feedback signal; the CS terminalcontroller connects and disconnects the CS terminal of the PWM controlIC to and from the negative power supply line according to the operationcontrol signal; the start-up corrector detects whether or not thefeedback signal is present so that, if the feedback signal is present,the start-up corrector connects, through a second resistor, the FBterminal of the PWM control IC to the negative power supply line and, ifnot, the start-up corrector cuts off the second resistor; and thestart-up switcher detects whether or not the feedback signal is presentso that, if the feedback signal is present, the start-up switcher turnson the supply of the operating power to the signal level checker and, ifnot, the start-up switcher turns off the supply of the operating powerto the signal level checker.

With this configuration, at the start of start-up, power immediatelystarts to be supplied to the PWM control IC, and the switching powersupply apparatus starts switching operation. This switching operationcauses the output voltage of the switching power supply apparatus toincrease until a feedback signal is generated, when the feedback signalis detected by the start-up corrector and the start-up switcher. As aresult, the start-up corrector connects the second resistor in additionto and in parallel with the first resistor, and the start-up switcherstarts to supply an operating current to the signal level checker.Supplied with the operating current, the signal level checker starts tooperate, and, during the period in which the signal level of thefeedback signal is lower than the voltage level of the current levelcheck reference power, the CS terminal controller keeps the CS terminalof the PWM control IC disconnected from the negative power supply lineso that operating power is kept supplied to the PWM control IC. Thus,switching operation is continued to permit the output voltage of theswitching power supply apparatus to increase to a predetermined value.

Thereafter, when the load of the switching power supply apparatus islight, and the signal level of the feedback signal is found to be higherthan the voltage level of the current level check reference power, theCS terminal controller connects the CS terminal of the PWM control IC tothe negative power supply line to turn off the supply of operating powerto the PWM control IC and thereby stop the switching operation of theswitching power supply apparatus. As the output voltage decreases, thesignal level of the feedback signal decreases until it becomes lowerthan the current level check reference, when the signal level checkerturns the operation control signal low. This causes the CS terminalcontroller to disconnect the CS terminal of the PWM control IC from thenegative power supply line so that operating power is supplied to thePWM control IC. This sequence of operations is repeated to achieve burstoscillation operation.

On the other hand, when the load of the switching power supply apparatusis heavy, and the signal level of the feedback signal does not reach thevoltage level of the current level check reference power, the signallevel checker turns the operation control signal low. This causes the CSterminal controller to disconnect the CS terminal of the PWM control ICfrom the negative power supply line so that continuous switchingoperation is continued.

In particular, the start-up corrector is so configured as to cut off thesecond resistor at start-up to increase the resistance between the FBterminal of the PWM control IC and the negative power supply line andthereby make the potential at the FB terminal higher. This ensuresreliable start-up operation. On the other hand, in the steady operation,the start-up corrector connects the second resistor in parallel with thefirst resistor to make the potential at the FB terminal of the PWMcontrol IC lower. This permits the PWM control IC to reliably controlthe switching power supply apparatus to output a stabilized voltage.

Preferably, the start-up switcher includes an NPN-type transistor havingthe collector thereof connected to the node between a current detectionresistor connected to the line of the feed back signal and the internalreference voltage line of the signal level checker, having the basethereof connected to the phototransistor, and having the emitter thereofconnected to the negative power supply line.

With this configuration, it is possible to realize the start-up switcherwith a simple circuit.

Preferably, the start-up corrector includes an NPN-type transistorhaving the collector thereof connected through the second resistor tothe FB terminal of the PWM control IC, having the base thereof connectedthrough a resistor to the phototransistor, and having the emitterthereof connected to the negative power supply line.

With this configuration, it is possible to realize the start-upcorrector with a simple circuit.

Preferably, when a PWM control IC is used as the switching controller, astart-up corrector is additionally provided to correct the start-up ofthe PWM control IC; a first resistor is connected between the FBterminal of the PWM control IC and the negative power supply line; thesignal level checker feeds a CS terminal controller, which serves as theoperation/nonoperation switcher, and the FB terminal with the operationcontrol signal and an inverted feedback signal, respectively, accordingto the result of checking of the signal level of the feedback signal;the CS terminal controller connects and disconnects the CS terminal ofthe PWM control IC to and from the negative power supply line accordingto the operation control signal; and the start-up corrector detectswhether or not the feedback signal is present so that, if the feedbacksignal is present, the start-up corrector connects, through a diode andthe second resistor, the FB terminal of the PWM control IC to thenegative power source line and turns on the supply of operating power tothe signal level checker and, if not, the start-up corrector cuts offthe diode and the second resistor and turns off the supply of theoperating power to the signal level checker.

With this configuration, at the start of start-up, power immediatelystarts to be supplied to the PWM control IC, and the switching powersupply apparatus starts switching operation. This switching operationcauses the output voltage of the switching power supply apparatus toincrease until a feedback signal is generated, when the feedback signalis detected by the start-up corrector. As a result, the start-upcorrector connects the second resistor in addition to and in parallelwith the first resistor, and starts to supply an operating current tothe signal level checker. Supplied with the operating current, thesignal level checker starts to operate, and, during the period in whichthe signal level of the feedback signal is lower than the voltage levelof the current level check reference power, the CS terminal controllerkeeps the CS terminal of the PWM control IC disconnected from thenegative power supply line so that operating power is kept supplied tothe PWM control IC. Thus, switching operation is continued to permit theoutput voltage of the switching power supply apparatus to increase to apredetermined value.

Thereafter, when the load of the switching power supply apparatus islight, and the signal level of the feedback signal is found to be higherthan the voltage level of the current level check reference power, theCS terminal controller connects the CS terminal of the PWM control IC tothe negative power supply line to turn off the supply of operating powerto the PWM control IC and thereby stop the switching operation of theswitching power supply apparatus. As the output voltage decreases, thesignal level of the feedback signal decreases until it becomes lowerthan the current level check reference, when the signal level checkerturns the operation control signal low. This causes the CS terminalcontroller to disconnect the CS terminal of the PWM control IC from thenegative power supply line so that operating power is supplied to thePWM control IC. This sequence of operations is repeated to achieve burstoscillation operation.

On the other hand, when the load of the switching power supply apparatusis heavy, and the signal level of the feedback signal does not reach thevoltage level of the current level check reference power, the signallevel checker turns the operation control signal low. This causes the CSterminal controller to disconnect the CS terminal of the PWM control ICfrom the negative power supply line so that continuous switchingoperation is continued.

In particular, the start-up corrector is so configured as to cut off thesecond resistor at start-up to increase the resistance between the FBterminal of the PWM control IC and the negative power supply line andthereby make the potential at the FB terminal higher. This ensuresreliable start-up operation. On the other hand, in the steady operation,the start-up corrector connects the second resistor in parallel with thefirst resistor to make the potential at the FB terminal of the PWMcontrol IC lower. This permits the PWM control IC to reliably controlthe switching power supply apparatus to output a stabilized voltage.

The diode prevents a current from flowing through the signal levelchecker in a predetermined timing period, and thereby prevents thesignal level checker from operating unnecessarily, contributing tohigher operation accuracy.

Preferably, the start-up corrector includes an NPN-type transistorhaving the collector thereof connected through the diode and the secondresistor to the FB terminal of the PWM control IC, having the basethereof connected through a resistor to the phototransistor, and havingthe emitter thereof connected to the negative power supply line.

With this configuration, it is possible to realize the start-upcorrector with a simple circuit.

Preferably, the signal level checker includes, for generation of thereference voltage, voltage division resistors, of which alower-potential-side resistor is divided into two resistors, with thenode therebetween connected through a diode to the CS terminalcontroller, and the CS terminal controller is connected through anotherdiode to the CS terminal of the PWM control IC.

With this configuration, by varying the resistances of the individualdivision resistors for generating the reference voltage, it is possibleto freely and accurately set the fluctuation width and burst switchingperiod of the output voltage in burst switching operation. Inparticular, by making the fluctuation width of the output voltage aswide as applications permit, it is possible to reduce unnecessary powerconsumption in burst switching operation.

Moreover, the other diode prevents a high-level voltage from beingapplied to the CS terminal of the PWM control IC when the switchingpower supply apparatus starts to start up. This is because, when ahigh-level voltage is applied to the CS terminal, the output of the PWMcontrol IC is turned off.

Preferably, when a PWM control IC is used as the switching controller, astart-up switcher is additionally provided to turn on and off the supplyof operating power to the signal level checker; a current adjuster isadditionally provided that is connected between the FB terminal of thePWM control IC and the negative power supply line to adjust the currentoutput from the FB terminal according to the signal level of thefeedback signal; the signal level checker feeds a CS terminalcontroller, which serves as the operation/nonoperation switcher, withthe operation control signal according to the result of checking of thesignal level of the feedback signal; the CS terminal controller connectsand disconnects the CS terminal of the PWM control IC to and from thenegative power supply line according to the operation control signal;and the start-up switcher detects whether or not the feedback signal ispresent so that, if the feedback signal is present, the start-upswitcher turns on the supply of operating power to the signal levelchecker and, if not, the start-up switcher turns off the supply ofoperating power to the signal level checker.

With this configuration, at the start-up of the switching power supplyapparatus, the current adjuster adjusts the current at the FB terminalof the PWM control IC. Thus, the PWM control IC makes the main switchingdevice perform switching operation with a great on-state duty, andthereby reduces the start-up time. Moreover, on detecting the feedbacksignal, the start-up switcher starts to supply operating power to thesignal level checker. Moreover, the CS terminal controller connects anddisconnects the CS terminal of the PWM control IC to and from thenegative power supply line according to the output signal of the signallevel checker, and thereby turns on and off the PWM control IC. In thisway, it is possible to realize burst switching operation with high poweruse efficiency.

BRIEF DESCRIPTION OF THE DRAWINGS

This and other objects and features of the present invention will becomeclear from the following description, taken in conjunction with thepreferred embodiments with reference to the accompanying drawings inwhich:

FIG. 1 is a circuit diagram of the switching power supply apparatus of afirst embodiment of the invention;

FIG. 2 is a circuit diagram of the switching power supply apparatus of asecond embodiment of the invention;

FIG. 3 is a circuit diagram of the switching power supply apparatus of athird embodiment of the invention;

FIG. 4 is a circuit diagram of the switching power supply apparatus of afourth embodiment of the invention;

FIG. 5 is a circuit diagram of a typical circuit configuration of aswitching power supply apparatus employing FA5511, for referencepurposes;

FIG. 6 is a circuit diagram showing an outline of the circuitconfiguration of FA5511;

FIG. 7 is a signal waveform diagram illustrating the start-up operationof the switching power supply apparatus shown in FIG. 5;

FIG. 8 is a signal waveform diagram illustrating the start-up operationof the switching power supply apparatus shown in FIG. 4;

FIG. 9 is a circuit diagram of the switching power supply apparatus of afifth embodiment of the invention;

FIG. 10 is a circuit diagram of the switching power supply apparatus ofa sixth embodiment of the invention;

FIG. 11 is a circuit diagram of the switching power supply apparatus ofa seventh embodiment of the invention;

FIG. 12 is a circuit diagram of the switching power supply apparatus ofan eighth embodiment of the invention;

FIG. 13 is a circuit diagram of the switching power supply apparatus ofa ninth embodiment of the invention;

FIG. 14 is a circuit diagram of the switching power supply apparatus ofa tenth embodiment of the invention;

FIG. 15 is a circuit diagram of the switching power supply apparatus ofan eleventh embodiment of the invention;

FIG. 16 is a circuit diagram of the switching power supply apparatus ofa twelfth embodiment of the invention;

FIG. 17 is a signal waveform diagram illustrating the start-up operationof the switching power supply apparatuses shown in FIGS. 16 and 19;

FIG. 18 is a circuit diagram of the switching power supply apparatus ofa thirteenth embodiment of the invention;

FIG. 19 is a circuit diagram of the switching power supply apparatus ofa fourteenth embodiment of the invention;

FIG. 20 is a circuit diagram of the switching power supply apparatus ofa fifteenth embodiment of the invention;

FIG. 21 is a circuit diagram of the switching power supply apparatus ofa sixteenth embodiment of the invention; and

FIG. 22 is a circuit diagram of the switching power supply apparatus ofa seventeenth embodiment of the invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Hereinafter, embodiments of the present invention will be described withreference to the drawings.

First Embodiment

FIG. 1 is a circuit diagram of the switching power supply apparatus of afirst embodiment of the invention.

In the switching power supply apparatus shown in FIG. 1, a transformer 3has its primary coil 4 connected, at its one end, to a positive powersupply line 1 and, at its other end, through a main switching device 5to a negative power supply line 2. The main switching device 5 isrealized with, for example, an FET (field-effect transistor). Thetransformer 3 has its secondary coil 6 connected, at its one end,through a diode 7 to an output line 25 and, at its other end, to anoutput line 26. Between the output lines 25 and 26, there are connecteda capacitor 45 and an output voltage detector circuit 9. The outputterminal of the output voltage detector circuit 9 is connected by way ofa line 9 a to the input terminal of a signal level checker circuit 15and to the input terminal of a switching controller circuit 19 to feedeach of them with a feedback signal.

An operating power source 16 has its negative end connected to thenegative power supply line 2, and has its positive end connected by wayof a line 16 a to the power terminal of the signal level checker circuit15 and to the input terminal of a switch circuit 17. The signal levelchecker circuit 15 feeds an operation control signal by way of a line 15a to the control terminal of the switch circuit 17. The output terminalof the switch circuit 17 is connected by way of a line 17 a to the powerterminal of the switching controller circuit 19. The output terminal ofthe switching controller circuit 19 is connected to the control terminalof the main switching device 5.

Next, the operation of the switching power supply apparatus of the firstembodiment will be described. When a voltage from a nonillustrateddirect-current power source is applied between the positive and negativepower supply lines 1 and 2, the main switching device 5, under thecontrol of the switching controller circuit 19, performs switchingoperation, and thereby causes a high-frequency current to flow throughthe primary coil 4 of the transformer 3. This induces a high-frequencyvoltage in the secondary coil 6 of the transformer 3. Thishigh-frequency voltage is rectified by the diode 7 and then smoothed bythe capacitor 45, and is thereby converted into a direct-currentvoltage. This direct-current voltage is applied between the output lines25 and 26 so as to be output as the output voltage of the switchingpower supply apparatus.

The output voltage detector circuit 9 compares the output voltagebetween the output lines 25 and 26 with a predetermined referencevoltage, and feeds the result of the comparison in the form of afeedback signal by way of the line 9 a to the signal level checkercircuit 15 and the switching controller circuit 19. The switchingcontroller circuit 19 operates from the power supplied thereto from theoperating power source 16 through the switch circuit 17, and, bycontrolling the timing with which the main switching device 5 is turnedon and off according to the feedback signal, performs control in such away that a desired direct-current voltage is output between the outputlines 25 and 26.

When the load connected between the output lines 25 and 26 (the outputterminals of the switching power supply apparatus) consumes a smallamount of electric power (i.e., in light-load operation), the outputvoltage between the output lines 25 and 26 (the output voltage of theswitching power supply apparatus) tends to be higher. To correct this,the output voltage detector circuit 9 outputs to the line 9 a a feedbacksignal having, for example, a higher level.

On the other hand, when the load connected between the output lines 25and 26 consumes a large amount of electric power (i.e., in heavy-loadoperation), the output voltage between the output lines 25 and 26 tendsto be lower. To correct this, the output voltage detector circuit 9outputs to the line 9 a a feedback signal having, for example, a lowerlevel.

When the output voltage between the output lines 25 and 26 (the outputvoltage of the switching power supply apparatus) is higher than thereference voltage, and the feedback signal output by way of the line 9 ahas a higher level, the signal level checker circuit 15 feeds anoperation control signal by way of the line 15 a to the switch circuit17 so as to turn the switch circuit 17 off.

With the switch circuit 17 turned off, the switching controller circuit19 ceases to be supplied with the voltage from the operating powersource 16, and thus stops operating. As a result, the main switchingdevice 5 stops operating, and thus permits the output voltage betweenthe output lines 25 and 26 (the output voltage of the switching powersupply apparatus) to decrease gradually.

As the output voltage decreases, the level of the feedback signal fromthe output voltage detector circuit 9 becomes, for example, lower. Then,the signal level checker circuit 15 feeds an operation control signal byway of the line 15 a to the switch circuit 17 so as to turn the switchcircuit 17 on. This causes the voltage from the operating power source16 to be supplied to the switching controller circuit 19, and thus theswitching controller circuit 19 restarts operating, and makes the mainswitching device 5 perform switching operation.

Consequently, the output voltage between the output lines 25 and 26 (theoutput voltage of the switching power supply apparatus) increases, andmeanwhile the output voltage detector circuit 9 feeds a higher-levelfeedback signal by way of the line 9 a to the signal level checkercircuit 15. Then, the signal level checker circuit 15 turns the switchcircuit 17 off to stop the operation of the switching controller circuit19 and thereby stop the switching operation by the main switching device5.

When the load connected between the output lines 25 and 26 is a heavyload that consumes a considerably large amount of electric power, theoutput voltage tends to be considerably low. In this case, the signallevel checker circuit 15 keeps the switch circuit 17 continuously on.Thus, the switching controller circuit 19 makes the main switchingdevice 5 perform switching operation continuously and thereby stabilizesthe output voltage.

As described above, bust switching operation is achieved by repeatedlystopping switching operation when the output voltage of the switchingpower supply apparatus increases and restarting switching operation whenthe output voltage decreases. This stabilizes the output voltage.

In burst switching operation, the operating power of the signal levelchecker circuit 15 is supplied thereto without passing through theswitch circuit 17, and therefore the signal level checker circuit 15keeps operating even when switching operation is being stopped. However,the power consumption of the signal level checker circuit 15 is farlower than that of the switching controller circuit 19, and accordinglythe switching power supply apparatus operates with less powerconsumption, contributing to energy saving.

Second Embodiment

FIG. 2 is a circuit diagram of the switching power supply apparatus of asecond embodiment of the invention.

In the switching power supply apparatus shown in FIG. 2, a transformer 3has its primary coil 4 connected, at its one end, to a positive powersupply line 1 and, at its other end, through a main switching device 5to a negative power supply line 2. The transformer 3 has its secondarycoil 6 connected, at its one end, through a diode 7 to an output line 25and, at its other end, to an output line 26. Between the output lines 25and 26, there are connected a capacitor 45 and an output voltagedetector circuit 9.

The output voltage detector circuit 9 is composed of two serial circuitsconnected between the output lines 25 and 26, more specifically onecomposed of a photocoupler 20, a resistor 21, and a shunt regulator 22and another composed of output voltage division resistors 23 and 24. Thephotocoupler 20 is composed of a photodiode 20 a and a phototransistor20 b. The control terminal of the shunt regulator 22 is connected to thenode between the output voltage division resistors 23 and 24. The shuntregulator 22 compares the voltage at the node between the output voltagedivision resistors 23 and 24 with a reference voltage that haspreviously been prepared internally, and permits a current commensuratewith the result of the comparison to flow through the photodiode 20 a.

An operating power source 16 has its negative end connected to thenegative power supply line 2, and has its positive end connected to asteady-state operating current supply line 16 a. A signal level checkercircuit 15 is composed of a Zener diode 191, a resistor 201, acomparator 18, and a current detection resistor 28.

The Zener diode 191 has its cathode connected to the steady-stateoperating current supply line 16 a, and has its anode connected to oneend of the resistor 201 and to the non-inverting input terminal of thecomparator 18. The other end of the resistor 201 is connected to thenegative power supply line 2. The current detection resistor 28 has itsone end connected to the steady-state operating current supply line 16a, and has its other end connected to the inverting input terminal ofthe comparator 18 and to the collector of the phototransistor 20 b ofthe photocoupler 20.

The comparator 18 has its positive power terminal connected to thesteady-state operating current supply line 16 a, and has its negativepower terminal connected to the negative power supply line 2. The outputterminal of the comparator 18 is connected by way of a line 15 a to thecontrol terminal of a switch circuit 17. The emitter of thephototransistor 20 b is connected by way of a line 19 a to the controlterminal of the switching controller circuit 19. The output terminal ofthe switching controller circuit 19 is connected to the control terminalof the main switching device 5.

Next, the operation of the switching power supply apparatus of thesecond embodiment will be described. When a direct-current voltage fromthe operating power source 16 is applied between the positive andnegative power supply lines 1 and 2, the main switching device 5, underthe control of the switching controller circuit 19, performs switchingoperation, and thereby causes a high-frequency current to flow throughthe primary coil 4 of the transformer 3. This induces a high-frequencyvoltage in the secondary coil 6 of the transformer 3. Thishigh-frequency voltage is rectified by the diode 7 and then smoothed bythe capacitor 45, and is thereby converted into a direct-currentvoltage. This direct-current voltage is applied between the output lines25 and 26 so as to be output as the output voltage of the switchingpower supply apparatus.

The output voltage detector circuit 9 compares the output voltagebetween the output lines 25 and 26 with a predetermined referencevoltage, and feeds the result of the comparison in the form of afeedback signal, on one hand, via the node between the collector of thephototransistor 20 b and the current detection resistor 28 to the signallevel checker circuit 15 and, on the other hand, by way of the line 19 ato the switching controller circuit 19.

More specifically, in the output voltage detector circuit 9, the shuntregulator 22 compares the voltage at the node between the output voltagedivision resistors 23 and 24 with a reference voltage that haspreviously been prepared internally, and makes a current commensuratewith the result of the comparison flow through the photodiode 20 a. Thephototransistor 20 b supplies a current commensurate with the currentflowing through the photodiode 20 a from the operating power source 16through current detection resistor 28 to the switching controllercircuit 19. Thus, according to the current supplied, the switchingcontroller circuit 19 controls the switching operation of the mainswitching device 5, and thereby controls the output voltage of theswitching power supply apparatus (the voltage between the output lines25 and 26) so as to make it equal to a predetermined value.

In the signal level checker circuit 15, the comparator 18 compares thevoltage drop across the current detection resistor 28 with the voltageof the current level check reference power generated by the Zener diode191 and the resistor 201, and feeds a signal commensurate with theresult of the comparison by way of the line 15 a to the switch circuit17. The Zener diode 191 may be replaced with a resistor.

When the load connected between the output lines 25 and 26 (the outputterminals of the switching power supply apparatus) consumes a smallamount of electric power (i.e., in light-load operation), the outputvoltage between the output lines 25 and 26 (the output voltage of theswitching power supply apparatus) tends to be higher. To correct this,the output voltage detector circuit 9 increases the current flowingthrough the phototransistor 20 b.

The comparator 18, as the result of comparing the current value throughthe phototransistor 20 b with the reference current level set by thecurrent level check reference power, outputs a high-level operationcontrol signal, and feeds this high-level operation control signal byway of the line 15 a to the control terminal of the switch circuit 17,which is thereby turned off. This stops the supply of the supply voltageto the switching controller circuit 19, and thus the switchingcontroller circuit 19 stops operating. Consequently, the main switchingdevice 5 stops operating, and thus the output voltage of the switchingpower supply apparatus decreases gradually.

As the output voltage decreases, the current value through thephototransistor 20 b decreases. Then, the comparator 18, as the resultof comparing the current value through the phototransistor 20 b with thereference current level set by the current level check reference power,outputs a low-level operation control signal, and feeds this low-leveloperation control signal by way of the line 15 a to the control terminalof the switch circuit 17, which is thereby turned on. This starts thesupply of the supply voltage to the switching controller circuit 19, andthus the switching controller circuit 19 starts to operate.Consequently, the main switching device 5 starts to operate, and thusthe output voltage of the switching power supply apparatus increasesgradually.

As the output voltage increases, the current value through thephototransistor 20 b increases. Then, the comparator 18, as the resultof comparing the current value through the phototransistor 20 b with thereference current level set by the current level check reference power,outputs a high-level operation control signal, and feeds this high-leveloperation control signal by way of the line 15 a to the control terminalof the switch circuit 17, which is thereby turned off. This stops thesupply of the supply voltage to the switching controller circuit 19, andthus the switching controller circuit 19 stops operating. Consequently,the main switching device 5 stops operating, and thus the output voltageof the switching power supply apparatus decreases gradually. Thereafter,this sequence of control is repeated, and burst oscillation is therebymaintained. In this way, the output voltage of the switching powersupply apparatus is kept approximately constant.

Incidentally, among the operations described above, those belonging tothe first part of the sequence described above, namely the turning offof the switch circuit 17, the stopping of the switching operation of themain switching device 5, the decrease in the output voltage, thedecrease in the current through the phototransistor 20 b, and the outputof the low-level signal from the comparator 18, are not performedsimultaneously, but, because of delays produced by various portions ofthe circuit, performing all these operations requires a certainoperation time, and, during this operation time, the switching powersupply apparatus stops switching operation.

Likewise, the operations belonging to the second part of the sequencedescribed above, namely the turning on of the switch circuit 17, thestarting of the switching operation of the main switching device 5, theincrease in the output voltage, the increase in the current through thephototransistor 20 b, and the output of the high-level signal from thecomparator 18, are not performed simultaneously, but, because of delaysproduced by various portions of the circuit, performing all theseoperations requires a certain operation time, and, during this operationtime, the switching power supply apparatus continues switchingoperation.

The theory that the operation times described above help maintain theperiods during which the switching power supply apparatus keepsperforming and stops performing switching operation applies not only inthis embodiment but also in the first embodiment.

A small degree of hysteresis may be introduced in the control byslightly lowering the voltage level of the current level check referencepower fed to the non-inverting input terminal of the comparator 18 atthe same time that the switch circuit 17 is turned off and, likewise,slightly raising the voltage level of the current level check referencepower fed to the non-inverting input terminal of the comparator 18 atthe same time that the switch circuit 17 is turned on. This helps makelonger the periods during which the switching power supply apparatuskeeps performing and stops performing switching operation.

On the other hand, when the load connected between the output lines 25and 26 consumes a large amount of electric power (i.e., in heavy-loadoperation), the output voltage between the output lines 25 and 26 tendsto be lower. This causes the current flowing through the phototransistor20 b to decrease, and thus causes the voltage drop across the currentdetection resistor 28 to become lower than the voltage across the Zenerdiode 191. Accordingly, the comparator 18 outputs a low-level operationcontrol signal, and thus the switch circuit 17 is kept continuously on,permitting the switching power supply apparatus to perform continuousswitching operation.

Here, it should be noted that burst switching is achieved according tothe result of comparison between the current value through thephototransistor 20 b and the reference current value (the voltage acrossthe Zener diode 191 as converted into a current value). The signal levelof the feedback signal from the output voltage detector circuit 9 (i.e.,the current value through the phototransistor 20 b) represents the loadcurrent value of the switching power supply apparatus. Thus, it ispossible to correctly set the load current value at which switchingbetween continuous switching operation and burst switching operation isperformed.

In burst switching operation, the output voltage fluctuates as describedearlier. However, since the signal level of the feedback signal, i.e.,the current value through the phototransistor 20 b, also represents theoutput voltage value of the switching power supply apparatus as hasalready been described and will also be described later, it is possibleto correctly set the upper and lower limits of the output voltage.

The signal level of the feedback signal may be detected on the line 19 aleading to the control terminal of the switching controller circuit 19.However, as will be described later, this configuration cannot cope witha case where a current flows out of the switching controller circuit 19via its control terminal. That is, as the supply of operating power tothe switching controller circuit 19 is turned on and off, the currentvalue flowing out of it via its control terminal varies, and thus thevoltage value at the control terminal no longer correctly represents theoutput voltage and the load current as described earlier.

In this way, burst switching operation is achieved by repeatedlystopping switching operation when the output voltage of the switchingpower supply apparatus increases and restarting stitching operation whenthe output voltage decreases. This helps stabilize the output voltage.

In burst switching operation, the operating power of the signal levelchecker circuit 15 is supplied thereto without passing through theswitch circuit 17, and therefore the signal level checker circuit 15keeps operating even when switching operation is being stopped. However,the power consumption of the signal level checker circuit 15 is farlower than that of the switching controller circuit 19, and accordinglythe switching power supply apparatus operates with less powerconsumption, contributing to energy saving.

Third Embodiment

FIG. 3 is a circuit diagram of the switching power supply apparatus of athird embodiment of the invention. FIG. 3 is a circuit diagram showingthe detailed circuit configuration of the operating power source 16shown in FIGS. 1 and 2. In FIG. 3, such circuit components that findtheir counterparts in FIGS. 1 and 2 are identified with the samereference numerals, and their explanations will not be repeated.

In FIG. 3, the operating power of the switching controller circuit 19 issupplied thereto by way of a start-up current supply line 29 a by way ofwhich a start-up current is supplied from the positive power supply line1 through a start-up resistor 29, or by way of a steady-state operatingcurrant supply line 16 a by way of which a voltage induced in asubsidiary coil 32 of the transformer 3 is supplied through a serialcircuit composed of a plurality of diodes 30 and 31. The operating powerof the signal level checker circuit 15 and the phototransistor 20 b ofthe photocoupler 20 is supplied thereto from subsidiary control powerextracted from the node between the diodes 30 and 31.

The circuit corresponding to the operating power source 16 describedearlier is composed of the subsidiary coil 32 of the transformer 3, thediode 31, a capacitor 33, the diode 30, the start-up resistor 29, and acapacitor 46. In this switching power supply apparatus, at the start ofstart-up, when a direct-current voltage from a nonillustrateddirect-current power source is applied between the positive and negativepower supply lines 1 and 2, a charge current flows through the capacitor46 by way of the start-up resistor 29, and, as will be described later,since the switch circuit 17 is on, when the charge voltage of thecapacitor 46 reaches a predetermined voltage level, the switchingcontroller circuit 19 starts to operate and starts to supply the mainswitching device 5 with a drive signal.

Thus, the switching power supply apparatus starts switching operation,and a high-frequency voltage is induced in the subsidiary coil 32 of thetransformer 3. This high-frequency voltage is rectified and smoothed bythe diode 31 and the capacitor 33, and is thereby converted into adirect-current voltage. The phototransistor 20 b and the comparator 18operate from the operating power supplied thereto from the capacitor 33,and operate in such a way as to keep the output voltage of the switchingpower supply apparatus at a predetermined value and achieve burstswitching control when the load is light as described earlier.

During the start-up operation of the switching power supply apparatus,the diode 30 prevents a current from flowing from the positive powersupply line 1 through the start-up resistor 29 to the capacitor 33, andthus helps shorten the time required for the charge voltage of thecapacitor 46 to reach the predetermined voltage level. On completion ofthe start-up of the switching power supply apparatus, the capacitor 46is charged mainly by the current fed thereto from the capacitor 33through the diode 30, and supplies operating power to the switchingcontroller circuit 19 through the switch circuit 17.

When the switching power supply apparatus starts to start up, the chargevoltage of the capacitor 33 is zero, and therefore the comparator 18 isnot operating. However, since the output terminal of this comparator 18is pulled down by a resistor 62, the switch circuit 17 is in an onstate.

Likewise, when the switching power supply apparatus starts to start up,the charge voltage of the capacitor 33 is zero, and therefore no currentflows through the phototransistor 20 b. Thus, the switching controllercircuit 19 controls the main switching device 5 on the assumption thatthe output voltage of the switching power supply apparatus is lower thanthe predetermined value. Thereafter, as the output voltage of theswitching power supply apparatus increases, the charge voltage of thecapacitor 33 increases until a current flows through the phototransistor20 b, when the switching power supply apparatus starts to operate in apredetermined steady state.

As described above, during the period after the switching power supplyapparatus starts to start up until it starts to operate in the steadystate, the switching controller circuit 19 and the switch circuit 17operates by using as operating power the charge voltage of the capacitor46. Accordingly, to prevent the charge voltage of the capacitor 46 frombecoming lower than the permitted minimum operating voltage during thatperiod, the capacitor 46 needs to be given a sufficiently highcapacitance.

By increasing the resistance of the start-up resistor 29, it is possibleto reduce the power loss through the start-up resistor 29. However,making the resistance too high results in lengthening the time requiredto charge the capacitor 46 when the switching power supply apparatusstarts up, slowing down its start-up.

In this embodiment, when the switching power supply apparatus starts up,the diode 30 prevents the charge accumulated in the capacitor 46 fromflowing out of it to the phototransistor 20 b and the comparator 18.This helps reduce the time required for start-up. Moreover, byincreasing the resistance of the start-up resistor 29, it is possible toreduce power consumption.

In the switching power supply apparatus of this embodiment, the signallevel checker circuit 15 achieves burst switching control by repeatedlyturning on and off the switch circuit 17 provided in the line by way ofwhich the switching controller circuit 19 is supplied with operatingpower. Moreover, in burst switching control, while the switchingoperation of the main switching device 5 is being stopped, the supply ofoperating power to the switching controller circuit 19 is also stopped.This helps reduce the power loss suffered while the switching operationis being stopped, and thus helps reduce the power consumption of theapparatus as a whole.

Fourth Embodiment

FIG. 4 is a circuit diagram of the switching power supply apparatus of afourth embodiment of the invention. In FIG. 4, such circuit componentsthat find their counterparts in FIGS. 1 to 3 are identified with thesame reference numerals, and their explanations will not be repeated.The switching power supply apparatus of this embodiment incorporates aPWM (pulse-width modulation) control IC, for example one with theproduct number FA5511 manufactured by Fuji Electric Co., Ltd. In FIG. 4,FA5511 is shown as an IC 38.

FIG. 6 shows an outline of the configuration of FA5511. In FIG. 6, whenoperating power is supplied to a Vcc terminal T6, this operating poweris supplied to an output buffer 101, an operation control circuit 102,and a 5 V voltage regulator 103. When the voltage supplied via the Vccterminal T6 becomes higher than a predetermined operation startingvoltage, the 5 V voltage regulator 103 is brought into an output-enabledstate, and thus supplies stabilized 5 V power, on one hand, by way of aninternal supply line 104 to a PWM logic circuit 105 and an OSC(oscillation circuit ) 106 and, on the other hand, by way of theinternal supply line 104 and then through a diode 107 and a resistor 108to an FB terminal T2.

An internal power terminal T7 is connected to the internal supply line104, and to this internal power terminal T7 is externally connected acapacitor 40 for eliminating noise from the internal supply line 104.This capacitor 40 prevents noise from being superimposed on the powersupplied by way of the internal supply line 104, and thereby preventserroneous control.

The oscillation frequency of the OSC 106 is set by the resistance of aresistor 36 that is externally connected via a terminal T1. Theoscillation signal generated by the OSC 106 is fed to the PWM logiccircuit 105. The FB terminal T2 is pulled up to the internal supply line104 through a serial circuit composed of a diode 107 and a resistor 108,and thus a voltage divided by the serial circuit and a circuit elementexternally connected to the FB terminal T2 is supplied to the PWM logiccircuit 105.

The PWM logic circuit 105 performs, in the manner that will be describedlater, logic calculation on the voltage level at the FB terminal T2, thevoltage level at a CS terminal T8, which will be described later, andthe oscillation signal fed from the OSC 106, and feeds the output buffer101 with a drive signal for driving the main switching device 5 (seeFIG. 4). The output buffer 101 current-amplifies the drive signal, andthen feeds it as the drive signal to the main switching device 5, whichis externally connected via an output terminal T5.

Via a terminal T3, a current detection signal from the main switchingdevice 5 is fed in. When the current flowing through the main switchingdevice 5 exceeds a predetermined level, the PWM logic circuit 105 shutsoff the drive signal for the main switching device 5 (reduces it to alow level) to protect the main switching device 5. A terminal T4 servesas a common ground terminal of the internal circuit of FA5511, and isconnected to the negative power supply line 2 (see FIG. 4) of theswitching power supply apparatus.

To a CS terminal T8 is externally connected a capacitor 41. At the sametime that the operation control circuit 102 outputs an output enablesignal to the 5 V voltage regulator 103 as described earlier, theoperation control circuit 102 feeds the capacitor 41 with a weakcurrent, with which the capacitor 41 is charged gradually. When theswitching power supply apparatus is operating in the steady state, theoperation control circuit 102 controls the charge voltage of thecapacitor 41 in such a way that it does not exceed a predeterminedvoltage level.

When the potential at the CS terminal T8 is forcibly turned low with anexternal circuit, the operation control circuit 102 disables the 5 Vvoltage regulator 103 and thereby stops the supply of power to theinternal supply line 104, and simultaneously the 5 V voltage regulator103 outputs a disable signal to the output buffer 101. Thus, when thepotential at the CS terminal T8 is forcibly turned low with an externalcircuit, the power consumption by FA5511 is greatly reduced.

The switching power supply apparatus of this embodiment exploits theabove-described function of FA5311. Specifically, when the outputvoltage of the switching power supply apparatus is high, and thus thesignal level of the feedback signal is high, the signal level checkercircuit 15 (see FIG. 4) forcibly turns the potential at the CS terminalT8 low with an external circuit, and thereby stops the operation of theoutput buffer 101, PWM logic circuit 105, and OSC 106. This causes theswitching power supply apparatus to stop operating, and as a result thesignal level of the feedback signal decreases. Then, the signal levelchecker circuit 15 ceases to forcibly turn the potential at the CSterminal T8 low, and thereby restarts the switching power supplyapparatus. In this way, in light-load operation of the switching powersupply apparatus, burst switching operation is achieved.

FIG. 5 shows, for reference proposes, the circuit configuration of aswitching power supply apparatus having a typical circuit configurationin a case where it adopts FA5511. In FIG. 5, such circuit componentsthat find their counterparts in FIG. 4 are identified with the samereference numerals, and their explanations will not be repeated. FIG. 7shows the waveforms of the signals observed at relevant points in theswitching power supply apparatus during the period after it starts tostart up until it starts to operate in the steady state. In FIG. 7, at(a) is shown the voltage 701 across the capacitor 46 shown in FIG. 5; at(b) are shown the voltage 702 at the FB terminal T2 of the IC 38, i.e.,FA5511, shown in FIG. 5, the oscillation signal 703 that the OSC 106(see FIG. 6) feeds to the PWM logic circuit 105 (see FIG. 6), and thevoltage 704 at the CS terminal T8; at (c) is shown the output signal 705output via the output terminal T5.

Now, with reference to FIGS. 5 and 7, the operation of this switchingpower supply apparatus will be described. First, when, at a time pointt0, a direct-current voltage is applied between the positive andnegative power supply lines 1 and 2, the voltage 701 across thecapacitor 46 increases gradually owing to a charge current suppliedthereto through the start-up resistor 29. When, at a time point t1, thevoltage reaches the predetermined operation starting voltage of FA5511,the voltage on the internal supply line 104 inside the IC 38 rises asdescribed earlier, and thus the OSC 106, PWM logic circuit 105, andoutput buffer 101 start to operate.

Thus, the OSC 106 feeds the PWM logic circuit 105 with an oscillationsignal 703 having constant upper and lower limits and a constant period,and, as a result of the capacitor 41 being charged with the weak currentfed thereto from the operation control circuit 102, the voltage 704 atthe CS terminal T8 increases gradually. At the time point t1, thevoltage between the output lines 25 and 26 is still zero, and thereforeno current flows through the shunt regulator 22 and the phototransistor20 b. Thus, the voltage 702 at the FB terminal T2 of the IC 38 is high.

When whichever of the voltage 704 at the CS terminal T8 and the voltage702 at the FB terminal T2 is lower is higher than the voltage of theoscillation signal 703 output from the OSC 106, the PWM logic circuit105 outputs an output signal (a pulse signal) 705 of which the level ishigher than the voltage at the output terminal T5 of the output buffer101. Thus, during the period from the time point t1 to a time point t2,during which the level of the voltage 704 at the CS terminal T8 is lowerthan the level of the oscillation signal 703 output from the OSC 106,the output signal 705 remains low. At the time point t2, when the levelof the voltage 704 at the CS terminal T8 momentarily exceeds the levelof the oscillation signal 703 of the OSC 106, the output signal 705becomes high and then remains high for the corresponding period, turningthe main switching device 5 on.

Thereafter, as the voltage 704 increases, the period during which theoutput signal 705 remains high becomes increasingly long, andcorrespondingly the power supplied from the secondary coil 6 of thetransformer 3 through the diode 7 to between the output lines 25 and 26increases. Thus, the voltage between the output lines 25 and 26increases until, at a time point t3, a current starts to flow throughthe shunt regulator 22 and the phototransistor 20 b, when the voltage702 at the FB terminal T2 starts to decrease.

Next, when, at a time point t5, the voltage 702 at the FB terminal T2becomes lower than the voltage 704 at the CS terminal T8, the periodduring which the output signal 705 at the output terminal T5 is high isdetermined by the result of comparison between the level of theoscillation signal 703 of the OSC 106 and the voltage 702 at the FBterminal T2. Since the level of the voltage 702 represents the feedbacksignal output from the output voltage detector circuit 9, the switchingpower supply apparatus now starts to operate in the steady state inwhich it outputs a predetermined voltage.

On the other hand, the charge voltage 701 of the capacitor 46 tends toslightly decrease during the period from the time point t1 to the timepoint t3, because during that period more current flows to the Vccterminal T6 than is supplied from the start-up resistor 29. However,this decrease is so controlled as not to go below the minimum operatingVcc voltage of the IC 38, i.e., FA5511, by giving the capacitor 46 asufficiently high capacitance.

As described earlier, the output voltage of the switching power supplyapparatus increases, and correspondingly the charge voltage 701 of thecapacitor 46 starts to increase at a time point t4 and reaches thesteady-state stable voltage at a time point t6.

It is to be understood that the circuit configuration of the switchingpower supply apparatus explained with reference to FIG. 5 is a mereexample of a typical circuit configuration in a case where FA5511 isadopted, and thus does not incorporate the function of achieving burstswitching in light-load operation which will be described later inconnection with this particular embodiment.

Next, the operation of the switching power supply apparatus of thisembodiment shown in FIG. 4 will be described with reference to a signalwaveform diagram shown in FIG. 8. In FIG. 8, at (a) is shown the voltage801 across the capacitor 46 shown in FIG. 4; at (b) are shown thevoltage 804 at the FB terminal T2 of the IC 38, i.e., FA5511, shown inFIG. 4, the oscillation signal 803 that the OSC 106 (see FIG. 6) feedsto the PWM logic circuit 105 (see FIG. 6), and the voltage 805 at the CSterminal T8 of the IC 38; at (c) is shown the output signal 806 outputvia the output terminal T5 of the IC 38.

First, when, at a time point T0, a direct-current voltage is appliedbetween the positive and negative power supply lines 1 and 2, thevoltage 801 across the capacitor 46 increases gradually owing to acharge current supplied thereto through the start-up resistor 29. When,at a time point T1, the voltage reaches the predetermined operationstarting voltage of FA5511, the voltage on the internal supply line 104inside the IC 38 rises as described earlier, and thus the OSC 106, PWMlogic circuit 105, and the output buffer 101 start to operate.

Thus, the OSC 106 feeds the PWM logic circuit 105 with an oscillationsignal 803 having constant upper and lower limits and a constant period,and, as a result of the capacitor 41 being charged with the weak currentfed thereto from the operation control circuit 102, the voltage 805 atthe CS terminal T8 increases gradually. At the time point T1, the chargevoltage of the capacitor 33 is zero, the output current of the signallevel checker circuit 15 is zero, and the switch of a start-up correctorcircuit 35 is, as will be described later, off. Accordingly, the voltage804 at the FB terminal T2 of the IC 38 is a division voltage thatresults from voltage division by the diode 107 provided inside the IC38, the resistor 108, and a resistor 39 a (see FIG. 4).

This division voltage has its value set to be slightly higher than thelower-limit voltage level of the oscillation signal 803.

When whichever of the voltage 805 at the CS terminal T8 and the voltage804 at the FB terminal T2 is lower is higher than the voltage level ofthe oscillation signal 803 output from the OSC 106, the PWM logiccircuit 105 outputs an output signal 806 via the output terminal T5 ofthe output buffer 101.

Thus, during the period from the time point T1 to a time point T2,during which the level of the voltage 805 at the CS terminal T8 is lowerthan the level of the oscillation signal 703 output from the OSC 106,the output signal 806 remains low. At the time point T2, when the levelof the voltage 805 at the CS terminal T8 momentarily exceeds the levelof the oscillation signal 803 of the OSC 106, the output signal 705becomes high and then remains high for the corresponding period, turningthe main switching device 5 on.

This causes the voltage between the output lines 25 and 26 to slightlyincrease, and thus causes the charge voltage of the capacitor 33 toincrease in such a way as to correspond to the increase in the voltagebetween the output lines 25 and 26. Consequently, a current starts to besupplied from the capacitor 33 through the signal level checker circuit15 to the FB terminal T2 of the IC 38, and thus the voltage 804 at theFB terminal T2 starts to increase.

When the current value through the phototransistor 20 b is lower than apredetermined value set within the signal level checker circuit 15, thesignal level checker circuit 15 supplies a current to the FB terminal T2of the IC 38; by contrast, when the current value through thephototransistor 20 b is higher than the predetermined value set withinthe signal level checker circuit 15, the signal level checker circuit 15feeds a current to a CS terminal controller circuit 37, but supplies nocurrent to either of the FB terminal T2 and the CS terminal T8.

While the signal level checker circuit 15 is supplying a current to theFB terminal T2 of the IC 38, when the current value through thephototransistor 20 b increases, the signal level checker circuit 15decreases the supply current (inverted feedback signal); by contrast,when the current value through the phototransistor 20 b decreases, thesignal level checker circuit 15 increases the supply current (invertedfeedback signal).

The supply current also depends on the operating power of the signallevel checker circuit 15; that is, it also depends on the charge voltageof the capacitor 33. Thus, as described earlier, after the switchingpower supply apparatus starts to start up, as the voltage between theoutput lines 25 and 26 increases, and thus as the charge voltage of thecapacitor 33 increases, the supply current increases.

Thereafter, when the supply current increases until the steady-operationstate is reached in which the voltage between the output lines 25 and 26are stabilized, the voltage between the output lines 25 and 26 and thecharge voltage of the capacitor 33 are stabilized at constant valuesdetermined by the predetermined output voltage of the switching powersupply apparatus and the winding ratio between the secondary coil 6 andthe subsidiary coil 32 of the transformer 3. Thus, now, the supplycurrent depends solely on the current value through the phototransistor20 b as described above.

Next, after the time point T2, as the voltage 805 at the CS terminal T8increases, the period during which the output signal 806 output via theoutput terminal T5 remains high becomes increasingly long, thus thevoltage between the output lines 25 and 26 increases, thus the chargevoltage of the capacitor 33 increases, and thus the current suppliedfrom the signal level checker circuit 15 increases. As a result of thiscourse of events, the voltage 804 at the FB terminal T2 increasesgradually.

After a time point T3, when the voltage 805 at the CS terminal T8becomes higher than the voltage 804 at the FB terminal T2, as describedearlier, the PWM logic circuit 105 compares the voltage 804 at the FBterminal T2 with the oscillation signal 803 of the OSC 106, and,according to the result of the comparison, outputs the output signal 806through the output buffer 101 via the output terminal T5 so as to feedthe output signal 806 as the drive signal to the main switching device5.

As described above, the charge voltage of the capacitor 33 depends onthe voltage between the output lines 25 and 26 and the winding ratiobetween the secondary coil 6 and subsidiary coil 32 of the transformer3. Thus, after the time point T2, as the voltage between the outputlines 25 and 26 increases, the charge voltage of the capacitor 33increases describing a curve 802 shown at (a) in FIG. 8. When, at a timepoint T4, the voltage of the capacitor 33 becomes higher than apredetermined value set within the start-up corrector circuit 35, thestart-up corrector circuit 35 turns on a switch provided therein so asto connect a resistor 39 b in parallel with the resistor 39 a.

Consequently, the voltage 804 at the FB terminal T2 momentarilydecreases, but, since the voltage level after this decrease is higherthan the lower limit of the oscillation signal 803, although thehigh-level period of the output signal 806 at the output terminal T5 ismomentarily shortened, the main switching device 5 continues switchingoperation. Thus, the voltage between the output lines 25 and 26 and thecharge voltage of the capacitor 33 still continues to increase, and thevoltage 804 at the FB terminal T2 starts to increase again.

Immediately before a time point T6, when the voltage resulting fromvoltage division by the resistors 23 and 24 reaches the comparisonreference value provided within the shunt regulator 22, a current startsto flow through the shunt regulator 22 , photodiode 20 a, andphototransistor 20 b. Thus, the supply current from the signal levelchecker circuit 15 stops increasing, the voltage 804 at the FB terminalT2 stops increasing, and the switching power supply apparatus starts tooperate in the steady state.

In this steady-state operation, for example, when the voltage betweenthe output lines 25 and 26 increases, the voltage resulting from voltagedivision by the resistors 23 and 24 increases, thus the current throughthe shunt regulator 22, photodiode 20 a, and phototransistor 20 bincreases, thus the supply current from the signal level checker circuit15 decreases, thus the voltage 804 at the FB terminal T2 decreases, thenthe PWM logic circuit 105 compares the oscillation signal 803 of the OSC106 with the voltage 804 at the FB terminal T2 and as a result outputsvia the output terminal T5 of the IC 38 an output signal (drive signal)806 of which the high-level period is short, thus the on-state duty ofthe main switching device 5 becomes shorter, and thus the currentsupplied through the diode 7 to the output line 25 decreases. As aresult of this course of events, the voltage between the output lines 25and 26 is decreased.

By contrast, when the voltage between the output lines 25 and 26decreases, the voltage resulting from voltage division by the resistors23 and 24 decreases, thus the current through the shunt regulator 22,photodiode 20 a, and phototransistor 20 b decreases, thus the supplycurrent from the signal level checker circuit 15 increases, thus thevoltage 804 at the FB terminal T2 increases, then the PWM logic circuit105 compares the oscillation signal 803 of the OSC 106 with the voltage804 at the FB terminal T2 and as a result outputs via the outputterminal T5 of the IC 38 an output signal (drive signal) 806 of whichthe high-level period is long, thus the on-state duty of the mainswitching device 5 becomes longer, and thus the current supplied throughthe diode 7 to the output line 25 increases. As a result of this courseof events, the voltage between the output lines 25 and 26 is increased.

Through this sequence of operations, the voltage between the outputlines 25 and 26 is stabilized at a predetermined value. Consequently,the charge voltage of the capacitor 33 is also stabilized, and thus theamount of current supplied from the signal level checker circuit 15depends solely on the current through the phototransistor 20 b.

The start-up corrector circuit 35 switches the resistance between the FBterminal T2 of the IC 38 and the negative power supply line 2 betweenwhen the switching power supply apparatus is starting up and when it isoperating in the steady state. This ensures that the switching powersupply apparatus reliably performs switching operation.

Specifically, when the switching power supply apparatus starts to startup, the charge voltage of the capacitor 33 is zero, and the currentsupplied from the signal level checker circuit 15 is zero. Thus, theresistor 39 a is given a high resistance so that, as described earlier,the voltage resulting from voltage division by the diode 107 providedwithin the IC 38, the resistor 108, and the resistor 39 a (see FIG. 4)is higher than the lower limit of the oscillation signal of the OSC 106.

If this is not the case, even after the voltage level at the CS terminalT8 of the IC 38 increases, the voltage level at the FB terminal T2remains lower than the lower limit of the oscillation signal of the OSC106, and thus the PWM logic circuit 105 does not output a high-levelsignal via the output terminal T5. This makes it impossible for theoutput voltage of the switching power supply apparatus to rise.

On the other hand, in steady-state operation, if the resistance betweenthe FB terminal T2 of the IC 38 and the negative power supply line 2 iskept high, for example, when the output voltage of the switching powersupply apparatus increases as a result of the switching power supplyapparatus operating in a no-load state, as the output voltage isstabilized, even when the signal level checker circuit 15 stops thesupply current, the voltage resulting from voltage division by the diode107 within the IC 38, the resistor 108, and the resistor 39 a (see FIG.4) does not fall below the lower limit of the oscillation signal of theOSC 106, and thus, quite inconveniently, the voltage at the FB terminalT2 cannot be so controlled as to decrease the output voltage.

To overcome this inconvenience, when the switching power supplyapparatus is starting up, as the charge voltage of the capacitor 33increases, immediately before a current starts to flow through thephototransistor 20 b, the signal level checker circuit 15 additionallyconnects the resistor 39 b so as to reduce the resistance between the FBterminal T2 of the IC 38 and the negative power supply line 2.

As described above, the switching power supply apparatus performs burstswitching operation in light-load operation. This helps reduce powerloss in light-load operation.

As described earlier, in the switching power supply apparatus, theoutput voltage tends to increase in light-load operation. To correctthis, the current value through the phototransistor 20 b is increased.This current through the phototransistor 20 b is made to flow throughthe current-detection resistor 34, and the voltage across thiscurrent-detection resistor 34 is compared with the reference voltageprovided within the signal level checker circuit 15 so that, when thevoltage across the current-detection resistor 34 is higher than thereference voltage, the signal level checker circuit 15 feeds the supplycurrent to the CS terminal controller circuit 37 and stops the supply ofcurrent to the FB terminal T2 of the IC 38.

On detecting the supply current, the CS terminal controller circuit 37turns on the switch provided therein to turn the voltage at the CSterminal T8 of the IC 38 low. When the voltage at the CS terminal T8 isturned low, the operation control circuit 102 turns off the output ofthe 5 V voltage regulator 103, and stops the supply of pull-up currentto the FB terminal T2 and the supply of operating power to the OSC 106and the PWM logic circuit 105.

Moreover, the operation control circuit 102 feeds a disable signal tothe output buffer 101 to stop the operation of the output buffer 101.This stops the feeding of the drive signal from the output terminal T5of the IC 38 to the main switching device 5, and thus the switchingpower supply apparatus stops switching operation.

Consequently, as the voltage between the output lines 25 and 26decreases, the voltage resulting from voltage division by the resistors23 and 24 decreases, thus the current flowing through the shuntregulator 22, photodiode 20 a, and phototransistor 20 b decreases, thusthe voltage across the current-detection resistor 34 decreases, and thenthe signal level checker circuit 15 compares the voltage across thecurrent-detection resistor 34 with the reference voltage providedtherein and judges the voltage across the current-detection resistor 34to be lower. Thus, the signal level checker circuit 15 feeds the supplycurrent to the FB terminal T2 of the IC 38 and stops the supply ofcurrent to the CS terminal controller circuit 37.

Consequently, the CS terminal controller circuit 37 turns off the switchprovided therein to turn the voltage at the CS terminal T8 of the IC 38high. Thus, the operation control circuit 102 turns the 5 V voltageregulator 103 on, and restarts the supply of pull-up current to the FBterminal T2 and the supply of operating power to the OSC 106 and the PWMlogic circuit 105. Moreover, the operation control circuit 102 feeds anenable signal to the output buffer 101 to restart the operation of theoutput buffer 101.

This restarts the supply of the drive signal from the output terminal T5of the IC 38 to the main switching device 5, and thus the switchingpower supply apparatus restarts switching operation.

Thereafter, when the voltage between the output lines 25 and 26increases again, switching operation is stopped as described above.When, consequently, the voltage between the output lines 25 and 26increases decreases, and the voltage across the current-detectionresistor 34 decreases, switching operation is restarted as describedabove. Through repetition of these operations, burst switching operationis achieved.

In this burst switching state, as the output current of the switchingpower supply apparatus is increased, the time comes when, during theperiod of switching operation, the voltage level across thecurrent-detection resistor 34 no longer reaches the level of thereference voltage provided within the signal level checker circuit 15.This is the start of a continuous switching mode.

Adopting the technique of the fourth embodiment described above makes itpossible to carry out the present invention on a practical basis simplyby adding an additional circuit to a commercially available PWM controlIC, for example one with the product number FA5511 manufactured by FujiElectric Co., Ltd. or an equivalent.

In the switching power supply apparatus of this embodiment, the signallevel checker circuit 15 achieves burst switching control by repeatedlyturning on and off the CS terminal controller circuit 37 provided in theline by way of which the IC 38, which serves as the switchingcontroller, is supplied with operating power. Moreover, in burstswitching control, while the switching operation of the main switchingdevice 5 is being stopped, the supply of operating power to theprincipal circuit portions of the IC 38 is also stopped. This helpsreduce the power loss suffered while the switching operation is beingstopped, and thus helps reduce the power consumption of the apparatus asa whole.

Here, the principal circuit portions of the IC 38 are the OSC 106, PWMlogic circuit 105, FB terminal T2, and output buffer 101.

Moreover, the start-up corrector circuit 35 so operates that, when theswitching power supply apparatus shifts from start-up operation tosteady-state operation, the resistor 39 b is connected in parallel withthe resistor 39 a to reduce the resistance between the FB terminal T2 ofthe IC 38 and the negative power supply line 2. This lowers thepotential at the FB terminal T2, and thereby ensures that the switchingpower supply apparatus performs reliable output voltage stabilizingcontrol when operating in the steady state.

Fifth Embodiment

FIG. 9 is a circuit diagram of the switching power supply apparatus of afifth embodiment of the invention. In FIG. 9, such circuit componentsthat find their counterparts in FIG. 4 are identified with the samereference numerals, and their explanations will not be repeated. In theswitching power supply apparatus of the fifth embodiment, theconfiguration of the signal level checker circuit 15, start-up correctorcircuit 35, and CS terminal controller circuit 37 is shown in detail.

The signal level checker circuit 15 is composed of PNP-type transistors47 and 48 and resistors 49, 50, and 51. The start-up corrector circuit35 is composed of a Zener diode 54, resistors 55, 56, and 39 b, and anNPN-type transistor 57. The CS terminal controller circuit 37 iscomposed of an NPN-type transistor 53 and a resistor 52. In thefollowing descriptions, both PNP-type and NPN-type transistors arereferred to simply as transistors.

In the signal level checker circuit 15, the emitter of the transistor 47and the emitter of the transistor 48 are connected together, and betweenthese emitters and the capacitor 33 is connected the resistor 49. Thebase of the transistor 47 is connected to the node between the emitterof the phototransistor 20 b and the current-detection resistor 34.

The base of the transistor 48 is connected to the point (the nodebetween the resistors 50 and 51) at which a reference voltage isgenerated by dividing the voltage across the capacitor 33 with theserially connected resistors 50 and 51. The collector of the transistor47 is connected to the FB terminal T2 of the IC 38, i.e., FA5511, andthe collector of the transistor 48 is connected to the base of thetransistor 53 provided in the CS terminal controller circuit 37.

Configured as described above, the signal level checker circuit 15operates in the following manner when the switching power supplyapparatus is operating in the steady state.

As described earlier, the charge voltage of the capacitor 33 isstabilized, and a voltage produced by dividing the charge voltage withthe resistors 50 and 51 is used as a reference voltage. Let thisreference voltage be Eb. At the node between the phototransistor 20 band the current-detection resistor 34, there appears a voltage roughlyproportional to the signal level of the feedback signal output from theoutput voltage detector circuit 9. When this voltage is lower than thereference voltage Eb, the transistor 47 is on and the transistor 48 isoff. Thus, a current Ia given by formula (1) below flows through thecollector of the transistor 47.Ia=(Ea−Ee−Va)/Rd  (1)

In formula (1) above, Ea represents the charge voltage of the capacitor33, Ee represents the voltage at the node between the phototransistor 20b and the current-detection resistor 34 (i.e., the base voltage of thetransistor 47), Va represents the forward voltage between the base andemitter of the transistor 47, and Rd represents the resistance of theresistor 49.

Accordingly, as the current flowing through the phototransistor 20 bincreases, the current supplied to the FB terminal T2 of the IC 38 isreduced, and, as the current flowing through the phototransistor 20 bdecreases, the current supplied to the FB terminal T2 is increased.Moreover, when the current flowing through the phototransistor 20 bfurther increases, the voltage at the node between the phototransistor20 b and the current-detection resistor 34 becomes higher than thereference voltage Eb. This turns the transistor 47 off and thetransistor 48 on, and thus a current is supplied from the collector ofthe transistor 48 to the CS terminal controller circuit 37.

Next, the CS terminal controller circuit 37 will be described. Thetransistor 53 has its collector connected to the CS terminal T8 of theIC 38, i.e., FA5511, has its emitter connected to the negative powersupply line 2, and has its base connected to the output end of thesignal level checker circuit 15.

Accordingly, when a current is supplied from the signal level checkercircuit 15, the transistor 53 turns on, and thereby turns the voltage atthe CS terminal T8 of the IC 38 low.

A diode 58 is connected between the CS terminal T8 of the IC 38 and thecapacitor 41, and this diode 58 serves to quicken the fluctuation of thevoltage level at the CS terminal T8 of the IC 38 and thereby quicken thespeed of switching between an oscillating state and a resting state inburst oscillation operation.

If this diode 58 is not inserted, i.e., if the CS terminal T8 of the IC38 is connected directly to the capacitor 41, when the transistor 53turns on, the voltage at the CS terminal T8 does not turn low until thecharge accumulated in the capacitor 41 is depleted. This delays thestopping of switching operation. On the other hand, when the transistor53 turns off, it takes time for the capacitor 41 to be charged by thecurrent supplied from the operation control circuit 102 to above thelower-limit voltage level of the oscillation signal of the OSC 106. Thisdelays the restarting of switching operation. As a result, in burstswitching operation, it occurs that, when the load current of theswitching power supply apparatus abruptly increases during the period inwhich switching operation is not being performed, the output voltagedecreases by an increased amount.

In applications where the effects of the delays in the stopping andrestarting of switching operation can be ignored, it is not necessary toinsert the diode 58.

Next, the start-up corrector circuit 35 will be described. In thestart-up corrector circuit 35, the Zener voltage of the Zener diode 54is the aforementioned predetermined voltage that is so set that, whenthe charge voltage of the capacitor 33 increases above it, thetransistor 57 is turned on. Accordingly, when the charge voltage of thecapacitor 33 increases above the Zener voltage (predetermined voltage),the transistor 57 is supplied with its base current from the capacitor33 through the Zener diode 54 and the resistor 55. This turns thetransistor 57 on, which thus connects the resistor 39 b in parallel withthe resistor 39 a and thereby lowers the voltage at the FB terminal T2of the IC 38.

In the switching power supply apparatus of this embodiment, the signallevel checker circuit 15 achieves burst switching control by repeatedlyturning on and off the CS terminal controller circuit 37 provided in theline by way of which the IC 38, which serves as the switchingcontroller, is supplied with operating power. Moreover, in burstswitching control, while the switching operation of the main switchingdevice 5 is being stopped, the supply of operating power to theprincipal circuit portions of the IC 38, namely the OSC 106, PWM logiccircuit 105, FB terminal T2, and output buffer 101, is also stopped.This helps reduce the power loss suffered while the switching operationis being stopped, and thus helps reduce the power consumption of theapparatus as a whole.

Moreover, the start-up corrector circuit 35 so operates that, when theswitching power supply apparatus shifts from start-up operation tosteady-state operation, the resistor 39 b is connected in parallel withthe resistor 39 a to reduce the resistance between the FB terminal T2 ofthe IC 38 and the negative power supply line 2. This lowers thepotential at the FB terminal T2, and thereby ensures that the switchingpower supply apparatus performs reliable output voltage stabilizingcontrol when operating in the steady state.

Moreover, the signal level checker circuit 15, start-up correctorcircuit 35, and CS terminal controller circuit 37 can be realized with asimple circuit configuration, and the switching controller circuit canbe realized with an IC 38, i.e., FA5511. This helps reduce the space ofthe circuit board, and thereby reduce the size and cost of the switchingpower supply apparatus.

Moreover, the switching controller circuit (IC 38) is separate from themain switching device 5, and therefore, as compared with a case wherethe main switching device is formed integrally in a single package (on asingle wafer) along with the switching controller circuit and othercomponents, it is possible to adopt a main switching device having a lowon-state resistance. This helps prevent degradation of power conversionefficiency in heavy-load operation. Incidentally, with the currenttechnology, forming the main switching device in a single package alongwith such other components results in giving the main switching device ahigh on-state resistance.

Sixth Embodiment

FIG. 10 is a circuit diagram of the switching power supply apparatus ofa sixth embodiment of the invention. In FIG. 10, such circuit componentsthat find their counterparts in FIG. 9 are identified with the samereference numerals, and their explanations will not be repeated.

In the switching power supply apparatus of the previous embodiment shownin FIG. 9, in burst switching operation, the period in which switchingoperation is stopped and the period in which switching operation isperformed depend, as described earlier, on the delays in the controlperformed by the output voltage control system. By contrast, in theswitching power supply apparatus of this embodiment shown in FIG. 10,the comparison reference power provided within the signal level checkercircuit 15 a is varied between in the period in which switchingoperation is stopped and in the period in which switching operation isperformed so that, according to how the width of this variation is set,the period in which switching operation is stopped and the period inwhich switching operation is performed can be extended and adjusted.

In this embodiment, to permit the setting of the comparison referencepower, in the voltage division circuit provided in the signal levelchecker circuit 15 shown in FIG. 9 and composed of the seriallyconnected resistors 50 and 51, the resistor 51 is divided into resistors51 a and 51 b as in the signal level checker circuit 15 a shown in FIG.10. The node between the resistors 51 a and 51 b is connected through adiode 59 to the CS terminal T8 of the IC 38.

In FIG. 10, when the switching power supply apparatus is operating inthe steady state, during the period in which switching operation isperformed, the voltage at the CS terminal T8 of the IC 38 is high, andthe diode 59 prevents a current from flowing from the CS terminal T8 tothe node between the resistors 51 a and 51 b. Thus, the base voltage ofthe transistor 48 (i.e., the comparison reference power) Esa is roughlyset as given by formula (2) below.Esa=[(Ra+Rb)×Ec]/(Ro+Ra+Rb)  (2)

In formula (2) above, Ra represents the resistance of the resistor 51 a,Rb represents the resistance of the resistor 51 b, Ro represents theresistance of the resistor 50, and Ec represents the charge voltage ofthe capacitor 33.

On the other hand, when the switching power supply apparatus isperforming burst switching operation, during the period in whichswitching operation is stopped, the transistor 53 is on, and theresistor 51 b is short-circuited. Thus, the base voltage of thetransistor 48 (i.e., the comparison reference power) Esb is roughly setas given by formula (3) below.Esb=(Ra×Ec)/(Ro+Ra)  (3)

Hence, the relationship Esa>Esb holds. By appropriately setting theresistances of the resistors 50, 51 a, and 51 b, it is possible to feelyset the value of Esa-Esb.

While the switching power supply apparatus is performing switchingoperation, when the output voltage increases as a result of, forexample, the load current decreasing, and thus the base voltage of thetransistor 47 increases above the voltage Esa, as described above, acurrent is supplied from the transistor 48 to the base of the transistor53. This turns the transistor 53 on, and thus the switching power supplyapparatus stops switching operation.

As a result, the output voltage of the switching power supply apparatusstarts to decrease, and, when the base voltage of the transistor 47decreases below the voltage Esb, the transistor 48 turns off and turnsthe voltage at the CS terminal T8 of the IC 38 high. Thus, the switchingpower supply apparatus restarts switching operation. As a result, theoutput voltage of the switching power supply apparatus increases, and,when the base voltage of the transistor 47 increases above the voltageEsa, the switching power supply apparatus stops switching operation.This sequence of operations is repeated.

Accordingly, the switching power supply apparatus that adopts the signallevel checker circuit 15 a shown in FIG. 10 exhibits operationcharacteristics as described below.

In the signal level checker circuit 15 shown in FIG. 9, the voltage ofthe comparison reference power is fixed. Thus, in a switching powersupply apparatus adopting this signal level checker circuit 15, when itis performing burst switching operation, the lengths of the period inwhich switching is performed and the period in which switching operationis stopped depend on the delay characteristics of the control performedby the output voltage control system. By contrast, in a switching powersupply apparatus adopting the signal level checker circuit 15 a shown inFIG. 10, the lengths of the period in which switching is performed andthe period in which switching operation is stopped are longer than inthe switching power supply apparatus adopting the signal level checkercircuit 15, and in addition those lengths can be freely set byappropriately setting the value of Esa-Esb as described earlier.

Moreover, in a switching power supply apparatus adopting the signallevel checker circuit 15 shown in FIG. 9, when it is performing burstswitching operation, the width of the fluctuation of the output voltage(the ripples in the output voltage) is set to be equal to the maximumvalue determined by the delay characteristics of the control of theoutput voltage control system. By contrast, in a switching power supplyapparatus adopting the signal level checker circuit 15 a shown in FIG.10, the width of the fluctuation of the output voltage is greater thanin the switching power supply apparatus adopting the signal levelchecker circuit 15, and in addition that width can be freely set byappropriately setting the value of Esa-Esb as described earlier.

Incidentally, increasing the width of the variation of the outputvoltage (i.e., the ripples in the output voltage) leads to the advantageof reducing the power loss suffered in burst switching operation.

Specifically, in a switching power supply apparatus adopting the signallevel checker circuit 15, switching operation is started when, in thestate in which switching operation is stopped, the signal level of thefeedback signal decreases even slightly. At this time point at whichswitching operation is started, the voltage at the FB terminal T2 of theIC 38 increases little, and therefore the duty of the drive signaloutput from the output terminal T5 of the IC 38 is small (i.e., thehigh-level period is short).

By contrast, in a switching power supply apparatus adopting the signallevel checker circuit 15 a, switching operation is not started until, inthe state in which switching operation is stopped, the signal level ofthe feedback signal decreases down to the level of the voltage Esb.Thus, at this time point at which switching operation is started, thevoltage at the FB terminal T2 of the IC 38 increases greatly, andtherefore the duty of the drive signal output from the output terminalT5 is great (i.e., the high-level period is long).

Accordingly, at the time point at which switching operation is started,a strikingly large current per switching period is fed from thesecondary coil 6 of the transformer 3 through the diode 7, and thus,when observed in a long time span, switching has only to be performed asmaller number of times. This helps reduce power loss.

Therefore, in applications where a minimum fluctuation width ispermitted in the output voltage in burst switching operation, the signallevel checker circuit 15 shown in FIG. 9 is adopted, and, inapplications where priority is given to reduction of power consumption,the signal level checker circuit 15 a shown in FIG. 10 is adopted. In acase where the signal level checker circuit 15 a is adopted, asdescribed earlier, the width of the fluctuation of the output voltagecan be set to be the optimum value that produces fluctuation smallerthan applications permit and that simultaneously minimizes powerconsumption.

Incidentally, when a switching power supply apparatus adopting thesignal level checker circuit 15 a is operating in a heavy-load state,its output voltage tends to decrease. This keeps the base voltage of thetransistor 47 lower than the voltage Esa, and thus the switching powersupply apparatus performs continuous switching.

In the switching power supply apparatus of this embodiment, the signallevel checker circuit 15 a achieves burst switching control byrepeatedly turning on and off the CS terminal controller circuit 37provided in the line by way of which the IC 38, which serves as theswitching controller, is supplied with operating power. Moreover, inburst switching control, while the switching operation of the mainswitching device 5 is being stopped, the supply of operating power tothe IC 38 is also stopped. This helps reduce the power loss sufferedwhile the switching operation is being stopped, and thus helps reducethe power consumption of the apparatus as a whole.

Moreover, the start-up corrector circuit 35 so operates that, when theswitching power supply apparatus shifts from start-up operation tosteady-state operation, the resistor 39 b is connected in parallel withthe resistor 39 a to reduce the resistance between the FB terminal T2 ofthe IC 38 and the negative power supply line 2. This lowers thepotential at the FB terminal T2, and thereby ensures that the switchingpower supply apparatus performs reliable output voltage stabilizingcontrol when operating in the steady state.

Moreover, the signal level checker circuit 15 a, start-up correctorcircuit 35, and CS terminal controller circuit 37 can be realized with asimple circuit configuration, and the switching controller circuit canbe realized with an IC 38, i.e., FA5511. This helps reduce the space ofthe circuit board, and thereby reduce the size and cost of the switchingpower supply apparatus.

Moreover, the switching controller circuit (IC 38) is separate from themain switching device 5, and therefore, as compared with a case wherethe main switching device is formed integrally in a single package (on asingle wafer) along with the switching controller circuit and othercomponents, it is possible to adopt a main switching device having a lowon-state resistance. This helps prevent degradation of power conversionefficiency in heavy-load operation. Incidentally, with the currenttechnology, forming the main switching device in a single package alongwith such other components results in giving the main switching device ahigh on-state resistance.

Seventh Embodiment

FIG. 11 is a circuit diagram of the switching power supply apparatus ofa seventh embodiment of the invention. In FIG. 11, such circuitcomponents that find their counterparts in FIG. 4 are identified withthe same reference numerals, and their explanations will not berepeated.

In the switching power supply apparatus shown in FIG. 4, the feedbacksignal is fed from the phototransistor 20 b through the signal levelchecker circuit 15 and then, after having the increase or decrease inits signal level inverted as described earlier, to the FB terminal T2 ofthe IC 38. By contrast, in the switching power supply apparatus of thisembodiment shown in FIG. 11, the feedback signal is fed from thephototransistor 20 b through the resistor 34 and a current adjustercircuit 60 to the FB terminal T2 of the IC 38. Moreover, in thisembodiment, the start-up corrector circuit is omitted for the reasonstated later.

The current adjuster circuit 60 absorbs from the FB terminal T2 of theIC 38 a current proportional to the voltage at the node between thephototransistor 20 b and the resistor 34. Accordingly, when the outputvoltage of the switching power supply apparatus is, for example, higherthan a predetermined value, the output voltage detector circuit 9increases the voltage at the node between the phototransistor 20 b andthe resistor 34, and the current adjuster circuit 60 increases, in amanner corresponding to the increase in that voltage, the current thatit absorbs from the FB terminal T2 of the IC 38. This causes the voltageat the FB terminal T2 to decrease.

As this voltage decreases, the PWM logic circuit 105 (see FIG. 6)provided within the IC 38 feeds, via the output terminal T5 of the IC38, the main switching device 5 with a drive signal of which thehigh-level period is short. This causes the current supplied from thesecondary coil 6 of the transformer 3 through the diode 7 to decrease,and thus the output voltage is so controlled as to decrease.

On the other hand, when the output voltage of the switching power supplyapparatus is, for example, lower than the predetermined value, theoutput voltage detector circuit 9 decreases the voltage at the nodebetween the phototransistor 20 b and the resistor 34, and the currentadjuster circuit 60 decreases, in a manner corresponding to the decreasein that voltage, the current that it absorbs from the FB terminal T2 ofthe IC 38. This causes the voltage at the FB terminal T2 to increase.

As this voltage increases, the PWM logic circuit 105 (see FIG. 6)provided within the IC 38 feeds, via the output terminal T5 of the IC38, the main switching device 5 with a drive signal of which thehigh-level period is long. This causes the current supplied from thesecondary coil 6 of the transformer 3 through the diode 7 to increase,and thus the output voltage is so controlled as to increase.

The principle on which the switching power supply apparatus shown inFIG. 11 achieves burst switching control is the same as that on whichthe switching power supply apparatus shown in FIG. 4 achieves burstswitching control.

The switching power supply apparatus of this embodiment, at start-up,starts up in the same manner as a common circuit (see FIG. 5) thatemploys FA5511 (IC 38). This makes it possible to omit the start-upcorrector circuit described earlier.

Specifically, at the time point t1 shown in FIG. 7, the charge voltageof the capacitor 33 is zero, the voltage at the node between thephototransistor 20 b and the resistor 34 is also zero, and the currentadjuster circuit 60 does not absorb a current from the FB terminal T2 ofthe IC 38. Thus, the voltage at the FB terminal T2 has the same level asthe output voltage of the 5 V voltage regulator 103 (see FIG. 6).

Thereafter, the output voltage of the switching power supply apparatusand the charge voltage of the capacitor 33 increase, and, when, at thetime point t3, the output voltage of the switching power supplyapparatus reaches close to the predetermined voltage set by theresistors 23 and 24, a current flows through the phototransistor 20 b.This causes the voltage at the node between the phototransistor 20 b andthe resistor 34 to increase, and the voltage at the FB terminal T2 ofthe IC 38 start to decrease. Now, control for outputting a stabilizedsteady-state voltage is started. During the period up to the time pointt3, the duty of the drive signal output from the output terminal T5 ofthe IC 38 is controlled by the voltage level at the CS terminal T8.

The operation described above is the same as the operation performed atstart-up by a common circuit employing FA5511 like the one described inconnection with the switching power supply apparatus shown in FIG. 4.The switching power supply apparatus shown in FIG. 11 does not require astart-up corrector circuit.

As described above, the switching power supply apparatus shown in FIG.11 does not require a start-up corrector circuit, and thus has anaccordingly simpler circuit configuration. However, in this switchingpower supply apparatus, as will be described later in connection withthe eighth embodiment, the characteristics of the individual componentssuch as semiconductor devices used in the current adjuster circuit 60drift with temperature. Disadvantageously, this causes variation in theload current target value at which switching is performed from burstswitching to normal continuous switching and the load current targetvalue at which switching is performed from normal continuous switchingto burst switching. Therefore, the switching power supply apparatusshown in FIG. 11 is suitable in applications where variation in the loadcurrent target—values is permitted.

In the switching power supply apparatus of this embodiment, the signallevel checker circuit 15 achieves burst switching control by repeatedlyturning on and off the CS terminal controller circuit. Moreover, inburst switching control, while the switching operation of the mainswitching device 5 is being stopped, the supply of operating power tothe principal components of the IC 38, namely the OSC 106, PWM logiccircuit 105, FB terminal T2, and output buffer 101, is also stopped.This helps reduce the power loss suffered while the switching operationis being stopped, and thus helps reduce the power consumption of theapparatus as a whole.

Moreover, when the switching power supply apparatus starts up, thecurrent adjuster circuit 60 so operates as to adjust the current at theFB terminal T2 of the IC 38. Thus, the PWM control IC makes the mainswitching device perform switching operation with a great on-state duty.This helps reduce start-up time.

Moreover, the switching controller circuit (IC 38) is separate from themain switching device 5, and therefore, as compared with a case wherethe main switching device is formed integrally in a single package (on asingle wafer) along with the switching controller circuit and othercomponents, it is possible to adopt a main switching device having a lowon-state resistance. This helps prevent degradation of power conversionefficiency in heavy-load operation.

Eighth Embodiment

FIG. 12 is a circuit diagram of the switching power supply apparatus ofan eighth embodiment of the invention. In FIG. 12, such circuitcomponents that find their counterparts in FIGS. 10 and 11 areidentified with the same reference numerals, and their explanations willnot be repeated.

In FIG. 12, the current adjuster circuit 60 is composed of a transistor70 and a resistor 72. The transistor 70 has its collector connected tothe FB terminal T2 of the IC 38, has its base connected to the nodebetween the resistor 34 and a resistor 71, and has its emitter connectedthrough the resistor 72 to the negative power supply line 2.

When the switching power supply apparatus is performing normalcontinuous switching operation, the voltage Ef at the FB terminal T2 ofthe IC 38 is roughly determined by formula (4) below.Ef=Er−(Ea−Vb)×Re/Rc−Vf  (4)

In formula (4) above, Er represents the output voltage of the 5 Vvoltage regulator 103 (see FIG. 6) provided within the IC 38, i.e.,FA5511, Ea represents the voltage at the node between the resistor 71and the resistor 34, Vb represents the forward voltage between the baseand emitter of the transistor 70, Re represents the resistance of thepull-up resistor 108 (see FIG. 6) provided within the IC 38, Rcrepresents the resistance of the resistor 72, and Vf represents theforward voltage drop across the diode 107 (see FIG. 6) provided withinthe IC 38.

As will be clear from formula (4) above, the voltage Ef relates to theforward voltage between the base and emitter of the transistor 70. Ingeneral, the forward voltage between the base and emitter of atransistor varies with temperature. Therefore, even when the basevoltage of the transistor 47 is stable, as the operating ambienttemperature varies, the forward voltage of the transistor 70 varies, andthus the voltage at the FB terminal T2 of the IC 38 varies.

Moreover, when the switching power supply apparatus is performingcontinuous switching, as described earlier, the voltage level at the FBterminal T2 of the IC 38 is varied according to the variation of theoutput voltage of the switching power supply apparatus so that theoutput voltage is stabilized. This means, since the output voltagedepends on the variation of the load, that the output voltage isstabilized by varying the voltage level at the FB terminal T2 accordingto the variation of the load current. Thus, the voltage value at the FBterminal T2 represents the load current of the switching power supplyapparatus.

In the switching power supply apparatus, as described earlier, incontinuous switching operation, as the load current is decreasedgradually, the base voltage of the transistor 47 increases, and, when itbecomes higher than the base voltage (comparison reference voltage) ofthe transistor 48, the transistor 53 turns on, achieving a shift intoburst switching operation. Thus, if the voltage at the FB terminal T2 ofthe IC 38 at the time point of this switching varies according to theoperating ambient temperature of the switching power supply apparatus,it follows that, quite undesirably, the load current at the time pointof that switching also varies according to the operating ambienttemperature of the switching power supply apparatus.

Depending on the type of the appliance connected to the switching powersupply apparatus, the appliance may require an accurate value as thereference relative to which to evaluate the load current to determinewhether to perform switching or not. Thus, in such applications, theswitching power supply apparatus is not very suitable. However, inapplications where such accuracy is not required, the switching powersupply apparatus, having a comparatively simple configuration, issuitable.

Incidentally, in the circuits shown in FIGS. 4, 9, and 10 describedearlier, instead of providing a current adjuster circuit 60, the causesfor the variation of the load current are eliminated. This makes itpossible to comparatively accurately evaluate the load current at thetime of operation mode switching. However, the additional provision ofthe start-up corrector circuit 35 makes the circuit configuration alittle more complicated.

In the switching power supply apparatus of this embodiment, the signallevel checker circuit 15 achieves burst switching control by repeatedlyturning on and off the CS terminal controller circuit 37 provided in theline by way of which the IC 38, which serves as the switchingcontroller, is supplied with operating power. Moreover, in burstswitching control, while the switching operation of the main switchingdevice 5 is being stopped, the supply of operating power to theprincipal circuit portions of the IC 38, namely the OSC 106, PWM logiccircuit 105, FB terminal T2, and output buffer 101, is also stopped.This helps reduce the power loss suffered while the switching operationis being stopped, and thus helps reduce the power consumption of theapparatus as a whole.

Moreover, when the switching power supply apparatus starts up, thecurrent adjuster circuit 60 so operates as to adjust the current at theFB terminal T2 of the IC 38. Thus, the PWM control IC makes the mainswitching device perform switching operation with a great on-state duty.This helps reduce start-up time.

Moreover, the signal level checker circuit 15, current adjuster circuit60, and CS terminal controller circuit 37 can be realized with a simplecircuit configuration, and the switching controller circuit can berealized with an IC 38, i.e., FA5511. This helps reduce the space of thecircuit board, and thereby reduce the size and cost of the switchingpower supply apparatus.

Moreover, the switching controller circuit (IC 38) is separate from themain switching device 5, and therefore, as compared with a case wherethe main switching device is formed integrally in a single package (on asingle wafer) along with the switching controller circuit and othercomponents, it is possible to adopt a main switching device having a lowon-state resistance. This helps prevent degradation of power conversionefficiency in heavy-load operation.

Ninth Embodiment

FIG. 13 is a circuit diagram of the switching power supply apparatus ofa ninth embodiment of the invention. In FIG. 13, such circuit componentsthat find their counterparts in FIG. 12 are identified with the samereference numerals, and their explanations will not be repeated.

The circuit shown in FIG. 13 differs from that shown in FIG. 12 in thata transistor 77 having identical characteristics with the transistor 70is additionally connected in series with the resistor 71. By the actionof this transistor 77, the circuit shown in FIG. 13 can alleviate driftof characteristics with temperature. Specifically, for example, when theoperating ambient temperature of the switching power supply apparatusrises, at the same time that the forward voltage between the base andemitter of the transistor 70 decreases, the forward voltage between thebase and emitter of the transistor 77 also decreases. This causes thebase voltage of the transistor 70 to decrease, and thereby suppressesvariation of the voltage at the FB terminal T2 of the IC 38.

In the switching power supply apparatus of this embodiment, the signallevel checker circuit 15 achieves burst switching control by repeatedlyturning on and off the CS terminal controller circuit 37 provided in theline by way of which the IC 38, which serves as the switchingcontroller, is supplied with operating power. Moreover, in burstswitching control, while the switching operation of the main switchingdevice 5 is being stopped, the supply of operating power to the IC 38 isalso stopped. This helps reduce the power loss suffered while theswitching operation is being stopped, and thus helps reduce the powerconsumption of the apparatus as a whole.

Moreover, when the switching power supply apparatus starts up, thecurrent adjuster circuit 60 so operates as to adjust the current at theFB terminal T2 of the IC 38. Thus, the PWM control IC makes the mainswitching device perform switching operation with a great on-state duty.This helps reduce start-up time.

Moreover, the signal level checker circuit 15, current adjuster circuit60, and CS terminal controller circuit 37 can be realized with a simplecircuit configuration, and the switching controller circuit can berealized with an IC 38, i.e., FA5511. This helps reduce the space of thecircuit board, and thereby reduce the size and cost of the switchingpower supply apparatus.

Moreover, the switching controller circuit (IC 38) is separate from themain switching device 5, and therefore, as compared with a case wherethe main switching device is formed integrally in a single package (on asingle wafer) along with the switching controller circuit and othercomponents, it is possible to adopt a main switching device having a lowon-state resistance. This helps prevent degradation of power conversionefficiency in heavy-load operation.

Tenth Embodiment

FIG. 14 is a circuit diagram of the switching power supply apparatus ofa tenth embodiment of the invention. In FIG. 14, such circuit componentsthat find their counterparts in FIG. 11 are identified with the samereference numerals, and their explanations will not be repeated.

The circuit shown in FIG. 14 differs from that shown in FIG. 11 in thata capacitor 75 is additionally connected between the terminal T7 and theFB terminal T2 of the IC 38. Moreover, in the circuit shown in FIG. 14,mainly for phase compensation of the output stabilizing control systemin the continuous switching state, a serial circuit composed of aresistor 73 and a capacitor 74 may be additionally connected between theFB terminal T2 of the IC 38 and the negative power supply line 2.

Adding the capacitor 74 and the resistor 73, however, causes thefollowing undesirable phenomenon in burst switching operation.

When the switching power supply apparatus is performing burst switchingoperation, during the period in which switching operation is stopped, asdescribed earlier, the output voltage of the 5 V voltage regulator 103(see FIG. 6) is zero, and thus the voltage at the FB terminal T2 of theIC 38 decreases. Thereafter, as described earlier, at the time point atwhich switching operation is started, the output voltage of the 5 Vvoltage regulator 103 rises, and thus a current flows into the capacitor74 through the diode 107 (see FIG. 6) and the resistor 108 (see FIG. 6).Thus, it takes a while for the voltage at the FB terminal T2 of the IC38 to reach the lower-limit voltage level of the output signal of theOSC 106. This delays the starting of switching operation.

For example, while the switching power supply apparatus is performingburst switching operation, when the load current abruptly increasesduring the period in which switching operation is stopped, the decreasein the output voltage of the switching power supply apparatus isdetected by the signal level checker circuit 15 detecting a decrease inthe feedback signal. In this case, even if the output voltage of the 5 Vvoltage regulator 103 (FIG. 6) is made to rise quickly, theaforementioned delay in the starting of switching operation, quitedisadvantageously, lets the output voltage of the switching power supplyapparatus further decrease during the delay.

That is, in burst switching operation, when the load increases abruptly,the delay in the operation of the burst switching operation controlsystem increases the amount by which the output voltage of the switchingpower supply apparatus decreases. For this reason, it is desirable toincrease the control speed of the burst switching control system as muchas possible.

Incidentally, when the output of the 5 V voltage regulator 103 (see FIG.6) rises, by supplying a current through the capacitor 75 to thecapacitor 74, it is possible to eliminate the delay of the operation ofthe burst switching operation control system. Moreover, by giving thecapacitor 75 a capacitance higher than that required to eliminate thedelay of the operation of the burst switching operation control system,it is possible to achieve the same effects as those achieved in theembodiment shown in FIG. 10.

Specifically, when the capacitor 75 is given so high a capacitance, inburst switching operation, at the time point at which switchingoperation is started, the voltage at the FB terminal T2 of the IC 38becomes higher than the value corresponding to the level of the feedbacksignal, and thus a drive signal having a great duty is fed via theoutput terminal T5 of the IC 38 to the main switching device 5. Thus,for the same reasons as stated in connection with the embodiment shownin FIG. 10, the switching power supply apparatus shown in FIG. 14contributes to reduction of the power loss suffered in the burstswitching operation.

As compared with the switching power supply apparatus of the embodimentshown in FIG. 10, however, the way how power loss in burst switchingoperation is reduced in the switching power supply apparatus of theembodiment shown in FIG. 14 is a little less reliable because of thedifficulty in setting the capacitance of the added capacitor 75. Thatis, in the switching power supply apparatus of the embodiment shown inFIG. 14, the addition of the capacitor 75 affects the phase compensationof the output voltage stabilizing control system, and therefore thisconfiguration can be suitably adopted in a case where, with thecapacitor 75 added, the desired phase compensation is achieved.

Incidentally, the addition of the capacitor 75 is effective also in acase where the capacitor 74 and the resistor 73 are added in theswitching power supply apparatus of the embodiment shown in FIGS. 4, 16,or 19.

In the switching power supply apparatus of this embodiment, the signallevel checker circuit 15 achieves burst switching control by repeatedlyturning on and off the CS terminal controller circuit 37 provided in theline by way of which the IC 38, which serves as the switchingcontroller, is supplied with operating power. Moreover, in burstswitching control, while the switching operation of the main switchingdevice 5 is being stopped, the supply of operating power to theprincipal components of the IC 38, namely the OSC 106, PWM logic circuit105, FB terminal T2, and output buffer 101, is also stopped. This helpsreduce the power loss suffered while the switching operation is beingstopped, and thus helps reduce the power consumption of the apparatus asa whole.

Moreover, when the switching power supply apparatus starts up, thecurrent adjuster circuit 60 so operates as to adjust the current at theFB terminal T2 of the IC 38. Thus, the PWM control IC makes the mainswitching device perform switching operation with a great on-state duty.This helps reduce start-up time.

Moreover, the switching controller circuit (IC 38) is separate from themain switching device 5, and therefore, as compared with a case wherethe main switching device is formed integrally in a single package (on asingle wafer) along with the switching controller circuit and othercomponents, it is possible to adopt a main switching device having a lowon-state resistance. This helps prevent degradation of power conversionefficiency in heavy-load operation.

Eleventh Embodiment

FIG. 15 is a circuit diagram of the switching power supply apparatus ofan eleventh embodiment of the invention. In FIG. 15, such circuitcomponents that find their counterparts in FIG. 14 are identified withthe same reference numerals, and their explanations will not berepeated.

The switching power supply apparatus shown in FIG. 15 differs from thatshown in FIG. 14 in that the phase compensation circuit composed of thecapacitor 74 and the resistor 73 which is provided in the latter isreplaced with two serial circuits in the former, specifically onecomposed of a capacitor 75 and a resistor 76 and another composed of acapacitor 78 and a resistor 79 as shown in FIG. 15. By giving thesecapacitors 75 and 78 and resistors 76 and 69 capacitances andresistances that fulfill formulae (5) and (6) below, it is possible tocompletely eliminate the effects of the phase compensation circuit inburst switching operation and to realize the desired phase compensationin continuous switching operation.Ca×Rm=Cb×Rn  (5)Ed=Er×Ca/(Ca+Cb)  (6)

In formulae (5) and (6) above, Ca represents the capacitance of thecapacitor 75, Cb represents the capacitance of the capacitor 78, Rmrepresents the resistance of the resistor 76, Rn represents theresistance of the resistor 79, Er represents the output voltage of the 5V voltage regulator 103 (see FIG. 6), and Ed represents the voltage drop(variation in voltage) that occurs at the FB terminal T2 of the IC 38.

More specifically, Ed represents the voltage drop (variation in voltage)that occurs at the FB terminal T2 of the IC 38 when the 5 V voltageregulator 103 stops its output in burst switching operation. Forexample, in the switching power supply apparatus of the embodiment shownin FIG. 15, immediately before the 5 V voltage regulator 103 stops itsoutput, the current adjuster circuit 60 absorbs from the FB terminal T2a current commensurate with the signal level of the feedback signal fedfrom the phototransistor 20 b, and thus the voltage at the FB terminalT2 is kept at the voltage level commensurate with the absorbed current.However, as soon as the 5 V voltage regulator 103 stops its output, thevoltage at the FB terminal T2 drops to zero. Ed represents this voltagedifference (voltage drop).

In formula (6), if the value of the right side is made greater than Ed,in burst switching operation, at the time point at which switchingoperation is started, the voltage at the FB terminal T2 becomes higherthan the value corresponding to the level of the feedback signal, andthus a drive signal having a great duty is fed via the output terminalT5 of the IC 38 to the main switching device 5. Thus, for the samereasons as stated in connection with the embodiment shown in FIG. 10,the switching power supply apparatus shown in FIG. 15 contributes toreduction of the power loss suffered in the burst switching operation.

Moreover, in the switching power supply apparatus of the embodimentshown in FIG. 15, by setting the capacitances of the capacitors and theresistances of the resistors in such a way that they fulfill formulae(7) and (8) below, it is possible, in continuous switching operation, toobtain the same phase compensation characteristic as when phasecompensation is achieved with the serial circuit composed of thecapacitor 74 and the resistor 73 (see FIG. 14) alone.Ca×Rm=Cb×Rn=Cd×Rt  (7)Cd=Ca+Cb  (8)

In formulae (7) and (8) above, Ca represents the capacitance of thecapacitor 75, Cb represents the capacitance of the capacitor 78, Cdrepresents the capacitance of the capacitor 74 (see FIG. 14), Rmrepresents the resistance of the resistor 76, Rn represents theresistance of the resistor 79, and Rt represents the resistance of theresistor 73.

In the switching power supply apparatus of this embodiment, the signallevel checker circuit 15 achieves burst switching control by repeatedlyturning on and off the CS terminal controller circuit 37 provided in theline by way of which the IC 38, which serves as the switchingcontroller, is supplied with operating power. Moreover, in burstswitching control, while the switching operation of the main switchingdevice 5 is being stopped, the supply of operating power to the IC 38 isalso stopped. This helps reduce the power loss suffered while theswitching operation is being stopped, and thus helps reduce the powerconsumption of the apparatus as a whole.

Moreover, when the switching power supply apparatus starts up, thecurrent adjuster circuit 60 so operates as to adjust the current at theFB terminal T2 of the IC 38. Thus, the PWM control IC makes the mainswitching device perform switching operation with a great on-state duty.This helps reduce start-up time.

Moreover, the switching controller circuit (IC 38) is separate from themain switching device 5, and therefore, as compared with a case wherethe main switching device is formed integrally in a single package (on asingle wafer) along with the switching controller circuit and othercomponents, it is possible to adopt a main switching device having a lowon-state resistance. This helps prevent degradation of power conversionefficiency in heavy-load operation.

Twelfth Embodiment

FIG. 16 is a circuit diagram of the switching power supply apparatus ofa twelfth embodiment of the invention. In FIG. 16, such circuitcomponents that find their counterparts in FIG. 4 are identified withthe same reference numerals, and their explanations will not berepeated.

In the switching power supply apparatus of this embodiment shown in FIG.16, the subsidiary control power used in FIG. 4 is omitted, and insteada direct current produced by rectifying with the diode 31 the voltageinduced in the subsidiary coil 32 of the transformer 3 is used as thepower of the control circuit, and is thus fed directly to the capacitor46.

When the switching power supply apparatus shown in FIG. 16 starts tostart up, as described earlier, the start-up current that is suppliedthrough the start-up resistor 29 flows through the signal level checkercircuit 15, and this lengthens the time required for the charge voltageof the capacitor 46 to reach the operation starting voltage of the IC38, i.e., FA5511. To prevent this, here, a start-up switcher circuit 81is additionally provided.

The output current (feedback signal) of the phototransistor 20 b is fedthrough a diode 80 to the signal level checker circuit 15, and thestart-up switcher circuit 81 and a start-up corrector circuit 82 checkwhether the feedback signal is present or not by monitoring the voltageat the node between the phototransistor 20 b and the diode 80.

The current consumed by the signal level checker circuit 15 (the currentconsumed including that consumed by the comparison reference power) isfed thereto from the positive terminal of the capacitor 46 by way of aline 84, and is returned by way of a line 83 through the switch providedwithin the start-up switcher circuit 81 to the negative terminal of thecapacitor 46. On the other hand, the current through the phototransistor20 b is fed thereto from the positive terminal of the capacitor 46, andis returned through the diode 80, the current-detection resistor 34, andthe switch provided within the start-up switcher circuit 81 to thenegative terminal of the capacitor 46.

When the switching power supply apparatus starts to start up, theinternal switch of the start-up switcher circuit 81 and the internalswitch of the start-up corrector circuit 82 are off, and the outputvoltage of the switching power supply apparatus is lower than apredetermined target voltage. Thus, no current is consumed by the signallevel checker circuit 15 (including the comparison reference powerprovided therein) or the phototransistor 20 b. Accordingly, the chargevoltage of the capacitor 46, owing to the start-up current fed theretothrough the start-up resistor 29, quickly rises and reaches theoperation start voltage level of the IC 38, i.e., FA5511. The timerequired for the charge voltage of the capacitor 46 to rise here isroughly as short as in a common configuration employing FF5511.

Next, the start-up operation of the switching power supply apparatuswill be described with reference to a signal waveform diagram shown inFIG. 17.

When, at a time point A0 shown at (a) in FIG. 17, a direct-currentvoltage is applied between the positive and negative power supply lines1 and 2, the voltage 213 across the capacitor 46 increases graduallyowing to the charge current supplied thereto through the start-upresistor 29. When, at a time point A1, the voltage 213 across thecapacitor 46 reaches the predetermined operation starting voltage of theIC 38, i.e., FA5511, the voltage on the internal supply line 104 withinthe IC 38 rises, and thus the OSC 106, PWM logic circuit 105, and outputbuffer 101 starts to operate.

Thus, the OSC 106 feeds the PWM logic circuit 105 with an oscillationsignal 214 having constant upper and lower limits and a constant period,and accordingly the voltage 216 at the CS terminal T8 of the IC 38increases gradually.

Moreover, at the time point A1, as described above, the start-up switch81 is off, and therefore the signal level checker circuit 15 is notsupplied with operating current. Thus, the output current of the signallevel checker circuit 15 is zero. Moreover, the switch of the start-upcorrector circuit 82 is off, and thus the voltage 215 at the FB terminalT2 of the IC 38 is equal to the division voltage resulting from voltagedivision by the diode 107 (see FIG. 6) provided within the IC 38, theresistor 108 (see FIG. 6), and the resistor 39 a (see FIG. 16). Here,the resistance of the resistor 39 a is so set that this division voltagehas roughly the same level as the upper-limit voltage level of theoscillation signal 214 of the OSC 106.

As described earlier, when whichever of the voltage 216 at the CSterminal T8 of the IC 38 and the voltage 215 at the FB terminal T2 islower is higher than the voltage level of the oscillation signal 214 ofthe OSC 106, the PWM logic circuit 105 outputs a high-level voltage viathe output terminal T5.

Accordingly, as shown at (c) in FIG. 17, during the period from the timepoint A0 to a time point A2, during which the level of the voltage 216at the CS terminal T8 of the IC 38 is lower than the voltage level ofthe oscillation signal 214 of the OSC 106, the voltage 217 at the outputterminal T5 of the IC 38 remains low. At the time point A2, when thevoltage level of the voltage 216 at the CS terminal T8 momentarilyexceeds the voltage level of the oscillation signal 214 of the OSC 106,the voltage 217 at the output terminal T5 becomes high and then remainshigh for the corresponding period, turning the main switching device 5on.

When the main switching device 5 is turned on in this way, the voltagebetween the output lines 25 and 26 slightly increases, and, during theperiod up to a time point A3, as the voltage 216 at the CS terminal T8of the IC 38 increases, the duty of the drive signal 217 output via theoutput terminal T5 of the IC 38 continues to increase. This causes theoutput voltage of the switching power supply apparatus to increasequickly.

When, at a time point A3, the output voltage of the switching powersupply apparatus reaches close to the predetermined target voltage,(i.e., when the voltage resulting from voltage division of the outputvoltage by the resistors 23 and 24 reaches a level roughly equal to thecomparison reference voltage within the shunt regulator 22), a currentflows through the shunt regulator 22 and the photodiode 20 a, and thusthe voltage at the node between the phototransistor 20 b and the diode80 increases. On detecting the increase in this voltage, the start-upswitcher circuit 81 turns its internal switch on to permit a current toflow through the signal level checker circuit 15 and the resistor 34.This causes the relevant circuits to start to operate.

On the other hand, the charge voltage of the capacitor 46, owing to thecurrent supplied thereto from the subsidiary coil 32 of the transformer3 through the diode 31, starts to increase immediately before the timepoint A3 at which the output voltage of the switching power supplyapparatus reaches the predetermined target voltage. At the time pointA3, the charge voltage of the capacitor 46 has reached the valuedetermined by the predetermined target output voltage of the switchingpower supply apparatus and the winding ratio between the subsidiary coil32 and secondary coil 6 of the transformer 3. Thus, the current flowingthrough the signal level checker circuit 15 and the resistor 34 preventsthe operating voltage of the IC 38 from falling below the permittedminimum operating voltage and thereby prevents its malfunctioning.

At the time point A3, the start-up corrector circuit 82 turns itsinternal switch on as does the start-up switcher circuit 81, and thusthe resistor 39 b is connected in parallel with the resistor 39 a. Now,as will be described later, the voltage 215 at the FB terminal T2 of theIC 38 starts to perform steady-state operation. It should be noted thatFIG. 17 illustrates an example in which the switching power supplyapparatus starts up in a heavy-load state, including what is shown inFIG. 8 described earlier.

When the current value through the phototransistor 20 b is lower than apredetermined value set within the signal level checker circuit 15, thesignal level checker circuit 15 supplies a current to the FB terminalT2; by contrast, when the current value through the phototransistor 20 bis higher than the predetermined value set within the signal levelchecker circuit 15, the signal level checker circuit 15 feeds a currentto the CS terminal controller circuit 37 to turn the internal switch ofthe CS terminal controller circuit 37 on, and stops the supply ofoperating power to the principal circuit portions of the IC 38, namelythe OSC 106, PWM logic circuit 105, FB terminal T2, and output buffer101.

Incidentally, the signal level checker circuit 15 does not supply acurrent simultaneously to the FB terminal T2 and the CS terminalcontroller circuit 37.

Moreover, while the signal level checker circuit 15 is supplying acurrent to the FB terminal T2 of the IC 38, the signal level checkercircuit 15 so functions as to decrease the supply current when thecurrent value through the phototransistor 20 b increases and increasethe supply current when the current value through the phototransistor 20b decreases. This function permits the output voltage of the switchingpower supply apparatus to be stabilized at the predetermined targetvalue.

Incidentally, the start-up corrector circuit 82 switches the resistancebetween the FB terminal T2 and the negative power supply line 2 betweenwhen the switching power supply apparatus is starting up and when it isoperating in the steady state. This ensures that the switching powersupply apparatus operates reliably.

Specifically, when the switching power supply apparatus starts to startup, the signal level checker circuit 15 is not operating, and thus nocurrent is supplied from the signal level checker circuit 15. Therefore,the resistor 39 a is given a high resistance so that the voltageresulting from voltage division by the diode 107 provided within the IC38, i.e., FA5511, the resistor 108 (see FIG. 6), and the resistor 39 a(see FIG. 16) is close to the upper limit of the oscillation signal 214of the OSC 106. If this is not the case, even after the voltage level216 at the CS terminal T8 of the IC 38 increases, the voltage level 215at the FB terminal T2 remains lower than the lower limit of theoscillation signal of the OSC 106, and thus the PWM logic circuit 105(see FIG. 6) does not output a high-level signal via the output terminalT5. This makes it impossible for the output voltage of the switchingpower supply apparatus to rise.

On the other hand, in steady-state operation, if the resistance betweenthe FB terminal T2 and the negative power supply line 2 is kept high,for example, when the output voltage of the switching power supplyapparatus increases as a result of the switching power supply apparatusoperating in a no-load state, as the output voltage is stabilized in themanner described earlier, even when the signal level checker circuit 15stops the supply current, the voltage at the FB terminal T2 does notfall below the lower-limit voltage of the oscillation signal 214 of theOSC 106 owing to the current supplied from the output line 104 of the 5V voltage regulator 103 (see FIG. 6) through the diode 107 and theresistor 108. Quite inconveniently, this makes it impossible to decreasethe output voltage of the switching power supply apparatus.

To overcome this inconvenience, when the switching power supplyapparatus starts up, at the time point A3, the start-up correctorcircuit 82 reduces the resistance between the FB terminal T2 and thenegative power supply line 2. After the time point A3, the internalswitches of the start-up switcher circuit 81 and the start-up correctorcircuit 82 are kept on, and thus, in light-load operation, the switchingpower supply apparatus performs burst switching operation on the sameprinciple as described in connection with the embodiment shown in FIG.4. This helps reduce power loss in light-load operation.

The switching power supply apparatus of this embodiment shown in FIG. 16permits omission of the capacitor 33 and the diode 30 used in theswitching power supply apparatus shown in FIG. 4, but instead requiresaddition of the start-up switcher circuit 81. While in this embodimentthe start-up switcher circuit 81 can easily be incorporated in an IC,the capacitor 33 cannot be incorporated in an IC. Thus, this embodimentis suitable to produce a new IC incorporating FA5511 or an equivalent ICalong with a CS terminal controller circuit, signal level checkercircuit, start-up switcher circuit, start-up corrector circuit, andother attendant circuits.

This embodiment is more susceptible than the embodiment shown in FIG. 4to the effects of temperature-related drift of the forward voltage ofthe diode 80, resulting in the disadvantage of a small degree oftemperature-related drift of the load current target value at whichswitching between burst switching and continuous switching is performed.Therefore, it is advisable to adopt the embodiment shown in FIG. 4 inapplications where the effects of temperature-related drift needs to bestrictly eliminated, and adopt the circuit of this embodiment inapplications where no such strict requirement needs to be met.

In the switching power supply apparatus of this embodiment, the signallevel checker circuit 15 achieves burst switching control by repeatedlyturning on and off the CS terminal controller circuit 37 provided in theline by way of which the IC 38, which serves as the switchingcontroller, is supplied with operating power. Moreover, in burstswitching control, while the switching operation of the main switchingdevice 5 is being stopped, the supply of operating power to theprincipal circuit portions of the IC 38, namely the OSC 106, PWM logiccircuit 105, FB terminal T2, and output buffer 101, is also stopped.This helps reduce the power loss suffered while the switching operationis being stopped, and thus helps reduce the power consumption of theapparatus as a whole.

Moreover, the start-up corrector circuit 35 so operates that the secondresistor 39 b is connected in parallel with the first resistor 39 a toreduce the resistance between the FB terminal T2 of the IC 38 and thenegative power supply line 2. This considerably lowers the potential atthe FB terminal T2, and thus the IC 38 can make the main switchingdevice 5 perform switching operation quickly. This helps reduce start-uptime.

Moreover, through the operation of the start-up switcher circuit 81,when the switching power supply apparatus starts to start up, thestart-up current supplied through the start-up resistor 29 is preventedfrom flowing through the signal level checker circuit 15 and therebylengthening the time required for the charge voltage of the capacitor 46to reach the operation starting voltage of the IC 38, i.e., FA5111.

Thirteenth Embodiment

FIG. 18 is a circuit diagram of the switching power supply apparatus ofa thirteenth embodiment of the invention. In FIG. 18, such circuitcomponents that find their counterparts in FIGS. 9 and 16 are identifiedwith the same reference numerals, and their explanations will not berepeated.

In FIG. 18, the signal level checker circuit 15 is composed of resistors49, 50, and 51 and transistors 47 and 48 The CS terminal controllercircuit 37 is composed of a resistor 52 and a transistor 53. Thestart-up switcher circuit 81 is composed of a resistor 85 and atransistor 84. The start-up corrector circuit 82 is composed ofresistors 87 and 39 b and a transistor 86.

As shown in FIG. 17 described earlier, when the switching power supplyapparatus starts to start up, at the time point A3, a current starts toflow through the phototransistor 20 b, and the voltage at the nodebetween the emitter of the phototransistor 20 b and the diode 80increases. This voltage is fed through the resistor 85 to the base ofthe transistor 84 and through the resistor 87 to the base of thetransistor 86, and thus the transistors 84 and 86 turn on. As the resultof the transistor 84 turning on, a current flows through the resistor 34and through the serial circuit composed of the resistors 50 and 51, anda base current starts to flow through the transistor 47. Thus, thesignal level checker circuit 15 starts to operate.

As described earlier, during the period up to the time point A3, nocurrent flows through the signal level checker circuit 15. This preventsthe lengthening of the time required for the charge voltage of thecapacitor 46 to reach the operation starting voltage of the IC 38, i.e.,FA5511.

Moreover, as the result of the transistor 86 turning on, the resistor 39b is added between the FB terminal T2 of the IC 38 and the negativepower supply line 2. This ensures that the switching power supplyapparatus performs reliable output voltage stabilizing control whenoperating in the steady state.

Incidentally, during the period up to the time point A3, the diode 80prevents the base currents of the transistors 47, 84, and 86 fromflowing along the route from the positive terminal of the capacitor 46to the resistor 49, to the emitter of the transistor 47, to the base ofthe transistor 47, to the resistor 85, to the base of the transistor 84,to the emitter of the transistor 84, to the negative power supply line2, and to the negative terminal of the capacitor 46 or along the routefrom the positive terminal of the capacitor 46 to the resistor 49, tothe emitter of the transistor 47, to the base of the transistor 47, tothe resistor 87, to the base of the transistor 86, to the emitter of thetransistor 86, to the negative power supply line 2, and to the negativeterminal of the capacitor 46. Thus, that this period, the diode 80prevents the transistors 47, 84, and 86 from being turned on, andthereby prevents the signal level checker circuit 15 from operating andthe resistor 39 b from being connected between the FB terminal T2 of theIC 38 and the negative power supply line 2.

In the switching power supply apparatus of this embodiment, the signallevel checker circuit 15 achieves burst switching control by repeatedlyturning on and off the CS terminal controller circuit 37 provided in theline by way of which the IC 38, which serves as the switchingcontroller, is supplied with operating power. Moreover, in burstswitching control, while the switching operation of the main switchingdevice 5 is being stopped, the supply of operating power to theprincipal circuit portions of the IC 38, namely the OSC 106, PWM logiccircuit 105, FB terminal T2, and output buffer 101, is also stopped.This helps reduce the power loss suffered while the switching operationis being stopped, and thus helps reduce the power consumption of theapparatus as a whole.

Moreover, the start-up corrector circuit 82 so operates that, when theswitching power supply apparatus shifts from start-up operation tosteady-state operation, the second resistor 39 b is connected inparallel with the first resistor 39 a to reduce the resistance betweenthe FB terminal T2 of the IC 38 and the negative power supply line 2.This lowers the potential at the FB terminal T2, and thereby ensuresthat the switching power supply apparatus performs reliable outputvoltage stabilizing control when operating in the steady state.

Moreover, through the operation of the start-up switcher circuit 81,when the switching power supply apparatus starts to start up, thestart-up current supplied through the start-up resistor 29 is preventedfrom flowing through the signal level checker circuit 15 and therebylengthening the time required for the charge voltage of the capacitor 46to reach the operation starting voltage of the IC 38, i.e., FA5111.

Moreover, the signal level checker circuit 15, start-up switcher circuit81, start-up corrector circuit 82, and CS terminal controller circuit 37can be realized with a simple circuit configuration, and the switchingcontroller circuit can be realized with an IC 38, i.e., FA5511. Thishelps reduce the space of the circuit board, and thereby reduce the sizeand cost of the switching power supply apparatus.

Moreover, the switching controller circuit (IC 38) is separate from themain switching device 5, and therefore, as compared with a case wherethe main switching device is formed integrally in a single package (on asingle wafer) along with the switching controller circuit and othercomponents, it is possible to adopt a main switching device having a lowon-state resistance. This helps prevent degradation of power conversionefficiency in heavy-load operation.

Fourteenth Embodiment

FIG. 19 is a circuit diagram of the switching power supply apparatus ofa fourteenth embodiment of the invention. In FIG. 19, such circuitcomponents that find their counterparts in FIG. 16 are identified withthe same reference numerals, and their explanations will not berepeated.

The switching power supply apparatus shown in FIG. 19 differs from thatshown in FIG. 16 in that the start-up switcher circuit 81 shown in FIG.16 is omitted and instead the feedback line 83 of the current consumedby the signal level checker circuit 15 is connected to the start-upcorrector circuit 82. The voltage waveforms observed at relevant pointsin the switching power supply apparatus during the period from the startof its start-up until a shift to steady-state operation are the same asin the switching power supply apparatus shown in FIG. 16, and thereforethe following description deals only with differences in operation.

In FIG. 17, when, at the time point A3, the internal switch of thestart-up corrector circuit 82 turns on, the operating current of thesignal level checker circuit 15 and the current though the resistor 34flow through the internal switch of the start-up corrector circuit 82,and thus the signal level checker circuit 15 starts to operate.Moreover, as the result of the internal switch of the start-up correctorcircuit 82 tuning on as described above, the resistor 39 b is connectedbetween the FB terminal T2 of the IC 38 and the negative power supplyline 2.

During the period from the time point A0 to the time point A3, a diode88 prevents a current from flowing along the route from the positiveterminal of the capacitor 46 to the operating current supply line 89 ofthe signal level checker circuit 15, to the signal level checker circuit15, to the resistor 34, to the resistor 39 b, to the resistor 39 a, tothe negative power supply line 2, and to the capacitor 46. Thus, duringthat period, the diode 88 prevents the signal level checker circuit 15from operating.

The switching power supply apparatus of this embodiment has a simplercircuit configuration than the switching power supply apparatus of theembodiment shown in FIG. 16. However, disadvantageously, the switchingpower supply apparatus of this embodiment is susceptible to thetemperature-related drift of the forward voltage drop across the diode88, and in addition the load current target value at which switchingbetween burst switching and continuous switching is performed increasesthe temperature-related drift. Thus, this configuration is suitable inapplications where the effects of temperature-drift can be ignored.

In the switching power supply apparatus of this embodiment, the signallevel checker circuit 15 achieves burst switching control by repeatedlyturning on and off the CS terminal controller circuit 37 provided in theline by way of which the IC 38, which serves as the switchingcontroller, is supplied with operating power. Moreover, in burstswitching control, while the switching operation of the main switchingdevice 5 is being stopped, the supply of operating power to the IC 38 isalso stopped. This helps reduce the power loss suffered while theswitching operation is being stopped, and thus helps reduce the powerconsumption of the apparatus as a whole.

Moreover, the start-up corrector circuit 82 so operates that, when theswitching power supply apparatus shifts from start-up operation tosteady-state operation, the second resistor 39 b is connected inparallel with the first resistor 39 a to reduce the resistance betweenthe FB terminal T2 of the IC 38 and the negative power supply line 2.This lowers the potential at the FB terminal T2, and thereby ensuresthat the switching power supply apparatus performs reliable outputvoltage stabilizing control when operating in the steady state.

Moreover, the switching controller circuit (IC 38) is separate from themain switching device 5, and therefore, as compared with a case wherethe main switching device is formed integrally in a single package (on asingle wafer) along with the switching controller circuit and othercomponents, it is possible to adopt a main switching device having a lowon-state resistance. This helps prevent degradation of power conversionefficiency in heavy-load operation.

Fifteenth Embodiment

FIG. 20 is a circuit diagram of the switching power supply apparatus ofa fifteenth embodiment of the invention. In FIG. 20, such circuitcomponents that find their counterparts in FIG. 18 are identified withthe same reference numerals, and their explanations will not berepeated.

As shown in FIG. 7, at the start of the start-up of the switching powersupply apparatus, when, at the time point A3, a current starts to flowthrough the phototransistor 20 b, and the voltage at the node betweenthe emitter of the phototransistor 20 b and the diode 80 increases, thisvoltage, through the base resistor 87, turns the transistor 86 on. Asthe result of the transistor 86 turning on, a current flows through theresistor 34 and through the serial circuit composed of the resistors 50and 51. This causes a base current to flow through the transistor 47,and thus the signal level checker circuit 15 starts to operate.

During the period up to the time point A3, no current flow through thesignal level checker circuit 15., This prevents the lengthening of thetime required for the charge voltage of the capacitor 46 to reach theoperation starting voltage of the IC 38, i.e., FA5511. Moreover, as theresult of the transistor 86 turning on, the serial circuit composed ofthe diode 88 and the resistor 39 b is added between the FB terminal T2of the IC 38 and the negative power supply line 2. This ensures that theswitching power supply apparatus performs reliable output voltagestabilizing control when operating in the steady state.

Incidentally, during the period up to the time point A3, the diode 80prevents the base currents of the transistors 47 and 86 from flowingalong the route from the positive terminal of the capacitor 46 to theresistor 49, to the emitter of the transistor 47, to the base of thetransistor 47, to the resistor 87, to the base of the transistor 86, tothe emitter of the transistor 86, to the negative power supply line 2,and to the negative terminal of the capacitor 46. Thus, during thatperiod, the diode 80 prevents the transistors 47 and 86 from beingturned on, and thereby prevents the signal level checker circuit 15 fromoperating and the resistor 39 b from being connected through the diode88 between the FB terminal T2 of the IC 38 and the negative power supplyline 2.

On the other hand, during the period from the time point A0 to the timepoint A3, the diode 88 prevents a current from flowing along the pathfrom the positive terminal of the capacitor 46 to the resistor 49, tothe emitter of the transistor 47, to the base of the transistor 47, tothe resistor 34, to the resistor 39 b, to the resistor 39 a, and to thenegative terminal of the capacitor 46. Thus, during that period, thediode 88 prevents the signal level checker circuit 15 from operating.

In the switching power supply apparatus of this embodiment, the signallevel checker circuit 15 achieves burst switching control by repeatedlyturning on and off the CS terminal controller circuit 37 provided in theline by way of which the IC 38, which serves as the switchingcontroller, is supplied with operating power. Moreover, in burstswitching control, while the switching operation of the main switchingdevice 5 is being stopped, the supply of operating power to theprincipal circuit portions of the IC 38, namely the OSC 106, PWM logiccircuit 105, FB terminal T2, and output buffer 101, is also stopped.This helps reduce the power loss suffered while the switching operationis being stopped, and thus helps reduce the power consumption of theapparatus as a whole.

Moreover, the start-up corrector circuit 82 so operates that, when theswitching power supply apparatus shifts from start-up operation tosteady-state operation, the second resistor 39 b is connected inparallel with the first resistor 39 a to reduce the resistance betweenthe FB terminal T2 of the IC 38 and the negative power supply line 2.This lowers the potential at the FB terminal T2, and thereby ensuresthat the switching power supply apparatus performs reliable outputvoltage stabilizing control when operating in the steady state.

Moreover, the signal level checker circuit 15, start-up correctorcircuit 82, and CS terminal controller circuit 37 can be realized with asimple circuit configuration, and the switching controller circuit canbe realized with an IC 38, i.e., FA5511. This helps reduce the space ofthe circuit board, and thereby reduce the size and cost of the switchingpower supply apparatus.

Moreover, the switching controller circuit (IC 38) is separate from themain switching device 5, and therefore, as compared with a case wherethe main switching device is formed integrally in a single package (on asingle wafer) along with the switching controller circuit and othercomponents, it is possible to adopt a main switching device having a lowon-state resistance. This helps prevent degradation of power conversionefficiency in heavy-load operation.

Sixteenth Embodiment

FIG. 21 is a circuit diagram of the switching power supply apparatus ofa sixteenth embodiment of the invention. In FIG. 21, such circuitcomponents that find their counterparts in FIGS. 10, 16, 18, and 19 areidentified with the same reference numerals, and their explanations willnot be repeated.

In the switching power supply apparatuses shown in FIGS. 16, 18, 19, and20, in burst switching operation, the period in which switchingoperation is stopped and the period in which switching operation isperformed depend, as described earlier, on the delays in the controlperformed by the output voltage control system. By contrast, in theswitching power supply apparatus of this embodiment shown in FIG. 21,like the switching power supply apparatus shown in FIG. 10, thecomparison reference voltage provided within the signal level checkercircuit 15 a is varied between in the period in which switchingoperation is stopped and in the period in which switching operation isperformed so that, according to how the width of this variation is set,the period in which switching operation is stopped and the period inwhich switching operation is performed can be extended and adjusted.

Specifically, in this embodiment, to obtain power corresponding to thecomparison reference power described earlier in connection with FIGS. 18and 20, in the voltage division circuit composed of the seriallyconnected resistors 50 and 51, the lower-potential-side resistor 51 isdivided into resistors 51 a and 51 b. The node between the resistors 51a and 51 b is connected through a diode 59 to the collector of thetransistor 53 provided in the CS terminal controller circuit 37, and thecollector of the transistor 53 is connected through a diode 90 to the CSterminal T8 of the IC 38.

If the diode 90 is not provided (i.e., if the collector of thetransistor 53 and the cathode of the diode 59 are connected directly tothe CS terminal T8 of the IC 38 without the diode 90 connected inbetween), when the switching power supply apparatus starts to start up,during the period in which the internal switches of the start-upswitcher circuit 81 and the start-up corrector circuit 82 are off, ahigh-level voltage is applied from the positive terminal of thecapacitor 46 through the resistors 50 and 5 la and the diode 59 to theCS terminal T8 of the IC 38. This turns off the output of the IC 38 viaits output terminal T5, and thus makes it impossible for the switchingpower supply apparatus to start up. This problem is overcome by theprovision of the diode 90.

In the switching power supply apparatus of this embodiment, the signallevel checker circuit 15 achieves burst switching control by repeatedlyturning on and off the CS terminal controller circuit 37 provided in theline by way of which the IC 38, which serves as the switchingcontroller, is supplied with operating power. Moreover, in burstswitching control, while the switching operation of the main switchingdevice 5 is being stopped, the supply of operating power to theprincipal circuit portions of the IC 38, namely the OSC 106, PWM logiccircuit 105, FB terminal T2, and output buffer 101, is also stopped.This helps reduce the power loss suffered while the switching operationis being stopped, and thus helps reduce the power consumption of theapparatus as a whole.

Moreover, the start-up corrector circuit 82 so operates that, when theswitching power supply apparatus shifts from start-up operation tosteady-state operation, the second resistor 39 b is connected inparallel with the first resistor 39 a to reduce the resistance betweenthe FB terminal T2 of the IC 38 and the negative power supply line 2.This lowers the potential at the FB terminal T2, and thereby ensuresthat the switching power supply apparatus performs reliable outputvoltage stabilizing control when operating in the steady state.

Moreover, through the operation of the start-up switcher circuit 81,when the switching power supply apparatus starts to start up, thestart-up current supplied through the start-up resistor 29 is preventedfrom flowing through the signal level checker circuit 1 Sa and therebylengthening the time required for the charge voltage of the capacitor 46to reach the operation starting voltage of the IC 38, i.e., FA5111.

Moreover, the signal level checker circuit 15 a, start-up switchercircuit 81, start-up corrector circuit 82, and CS terminal controllercircuit 37 can be realized with a simple circuit configuration, and theswitching controller circuit can be realized with an IC 38, i.e.,FA5511. This helps reduce the space of the circuit board, and therebyreduce the size and cost of the switching power supply apparatus.

Moreover, the switching controller circuit (IC 38) is separate from themain switching device 5, and therefore, as compared with a case wherethe main switching device is formed integrally in a single package (on asingle wafer) along with the switching controller circuit and othercomponents, it is possible to adopt a main switching device having a lowon-state resistance. This helps prevent degradation of power conversionefficiency in heavy-load operation.

Seventeenth Embodiment

FIG. 22 is a circuit diagram of the switching power supply apparatus ofa seventeenth embodiment of the invention. In FIG. 22, such circuitcomponents that find their counterparts in FIGS. 11 and 16 areidentified with the same reference numerals, and their explanations willnot be repeated. The switching power supply apparatus of this embodimentshown in FIG. 22 omits the start-up corrector circuit 82 shown in FIG.16, and is instead additionally provided with a current adjuster circuit60 shown in FIG. 11.

In the switching power supply apparatus of this embodiment shown in FIG.22, a feedback signal is fed from the phototransistor 20 b through thediode 80, the resistor 34, and the current adjuster circuit 60 to the FBterminal T2 of the IC 38. The current adjuster circuit 60 absorbs fromthe FB terminal T2 of the IC 38 a current proportional to the voltage atthe node between the diode 80 and the resistor 34.

Accordingly, when the output voltage of the switching power supplyapparatus is, for example, higher than a predetermined value, the outputvoltage detector circuit 9 increases the voltage at the node between thediode 80 and the resistor 34, and the current adjuster circuit 60increases, in a manner corresponding to the increase in that voltage,the current that it absorbs from the FB terminal T2 of the IC 38. Thiscauses the voltage at the FB terminal T2 to decrease.

As this voltage decreases, the PWM logic circuit 105 (see FIG. 6)provided within the IC 38 feeds, via the output terminal T5 of the IC38, the main switching device 5 with a drive signal of which thehigh-level period is short. This causes the current supplied from thesecondary coil 6 of the transformer 3 through the diode 7 to decrease,and thus the output voltage is so controlled as to decrease.

On the other hand, when the output voltage of the switching power supplyapparatus is, for example, lower than the predetermined value, theoutput voltage detector circuit 9 decreases the voltage at the nodebetween the diode 80 and the resistor 34, and the current adjustercircuit 60 decreases, in a manner corresponding to the decrease in thatvoltage, the current that it absorbs from the FB terminal T2 of the IC38. This causes the voltage at the FB terminal T2 to increase.

As this voltage increases, the PWM logic circuit 105 (see FIG. 6)provided within the IC 38 feeds, via the output terminal T5 of the IC38, the main switching device 5 with a drive signal of which thehigh-level period is long. This causes the current supplied from thesecondary coil 6 of the transformer 3 through the diode 7 to increase,and thus the output voltage is so controlled as to increase.

When the switching power supply apparatus starts to start up, asdescribed earlier, the start-up current that is supplied through thestart-up resistor 29 flows through the signal level checker circuit 15,and this lengthens the time required for the charge voltage of thecapacitor 46 to reach the operation starting voltage of the IC 38, i.e.,FA5511. To prevent this, here, a start-up switcher circuit 81 isadditionally provided.

The output current (feedback signal) of the phototransistor 20 b is fedthrough a diode 80 to the signal level checker circuit 15, and thestart-up switcher circuit 81 checks whether the feedback signal ispresent or not by monitoring the voltage at the node between thephototransistor 20 b and the diode 80.

The current consumed by the signal level checker circuit 15 (the currentconsumed including that consumed by the comparison reference power) isfed thereto from the positive terminal of the capacitor 46 by way of aline 84, and is returned by way of a line 83 through the switch providedwithin the start-up switcher circuit 81 to the negative terminal of thecapacitor 46. On the other hand, the current through the phototransistor20 b is fed thereto from the positive terminal of the capacitor 46, andis returned through the diode 80, the current-detection resistor 34, andthe switch provided within the start-up switcher circuit 81 to thenegative terminal of the capacitor 46.

When the switching power supply apparatus starts to start up, theinternal switch of the start-up switcher circuit 81 is off, and theoutput voltage of the switching power supply apparatus is lower than apredetermined target voltage. Thus, no current is consumed by the signallevel checker circuit 15 (including the comparison reference powerprovided therein) or the phototransistor 20 b. Accordingly, the chargevoltage of the capacitor 46, owing to the start-up current fed theretothrough the start-up resistor 29, quickly rises and reaches theoperation start voltage level of the IC 38, i.e., FA5511.

In the switching power supply apparatus of this embodiment, the signallevel checker circuit 15 achieves burst switching control by repeatedlyturning on and off the CS terminal controller circuit 37 provided in theline by way of which the IC 38, which serves as the switchingcontroller, is supplied with operating power. Moreover, in burstswitching control, while the switching operation of the main switchingdevice 5 is being stopped, the supply of operating power to theprincipal circuit portions of the IC 38, namely the OSC 106, PWM logiccircuit 105, FB terminal T2, and output buffer 101, is also stopped.This helps reduce the power loss suffered while the switching operationis being stopped, and thus helps reduce the power consumption of theapparatus as a whole.

Moreover, when the switching power supply apparatus starts up, thecurrent adjuster circuit 60 so operates as to adjust the current at theFB terminal T2 of the IC 38. Thus, the PWM control IC makes the mainswitching device perform switching operation with a great on-state duty.This helps reduce start-up time.

Moreover, through the operation of the start-up switcher circuit 81,when the switching power supply apparatus starts to start up, thestart-up current supplied through the start-up resistor 29 is preventedfrom flowing through the signal level checker circuit 15 and therebylengthening the time required for the charge voltage of the capacitor 46to reach the operation starting voltage of the IC 38, i.e., FA5111.

In the embodiments described hereinbefore, FA5511 manufactured by FujiElectric Co., Ltd. is used as the switching controller. However, it isalso possible to use any other IC having equivalent functions to realizesimilar circuit configurations.

In the switching power supply apparatus disclosed in Japanese PatentApplication Laid-Open No. H10-304658 mentioned as prior art, thestart-up circuit needs to adopt a control device resistant to a highvoltage to shut off a voltage (at the drain of an FET serving as themain switching device) obtained by rectifying and smoothing commerciallydistributed alternating-current power. Disadvantageously, this increasesthe costs of this switching power supply apparatus. To overcome thisdisadvantage, the start-up circuit adopts a structure in which the mainswitching device is formed in a single package along with othercomponents including the control device. However, with the currenttechnology, it is impossible to form a main switching device with a lowon-state resistance in a single package along with such othercomponents. This leads to lower power conversion efficiency when theswitching power supply apparatus is operating in a heavy-load state.

To solve this problem, in the switching power supply apparatuses of theembodiments described hereinbefore, the switching controller isseparated from the main switching device. This makes it possible to usea main switching device having a low on-state resistance and therebyachieve high power conversion efficiency.

As described above, according to the present invention, a switchingpower supply apparatus uses as a feedback signal the result ofcomparison between the output direct-current voltage and a predeterminedreference voltage, and drives the main switching device by turning onand off, according to the signal level of the feedback signal, thesupply of operating power to a main switching device driving system thatdrives the main switching device. Thus, while the switching operation ofthe main switching device is being stopped in burst switching control,the supply of operating power to the main switching device drivingsystem is also stopped. This helps reduce the power loss suffered whilethe switching operation is being stopped, and thus helps reduce thepower consumption of the apparatus as a whole.

According to the present invention, a switching power supply apparatusincludes: an output voltage detector that compares the outputdirect-current voltage with a predetermined reference voltage and thatoutputs the result of the comparison as a feedback signal; a switchingcontroller that drives and controls the main switching device accordingto the feedback signal output from the output voltage detector; a signallevel checker that monitors the signal level of the feedback signal andthat outputs an operation control signal for turning on and off theswitching controller according to the monitored signal level; and anoperation/nonoperation switcher that is provided in the line by way ofwhich the switching controller is supplied with operating power and thatturns on and off the switching controller according to the operationcontrol signal from the signal level checker. Thus, burst switchingcontrol is achieved as a result of the signal level checker repeatedlyturning on and off the operation/nonoperation switcher provided in theline by way of which the switching controller is supplied with operatingpower. Moreover, while the switching operation of the main switchingdevice is being stopped in burst switching control, the supply of theoperating power to the switching controller is also stopped. This helpsreduce the power loss suffered while the switching operation is beingstopped, and thus helps reduce the power consumption of the apparatus asa whole.

According to the present invention, in a switching power supplyapparatus, the signal level of the feedback signal is compared with thesignal level of a previously generated oscillation signal, so that,according to the result of the comparison, the on-state duty of thedrive signal to be fed to the main switching device is determined andswitching between burst switching control and continuous switchingcontrol is performed. Moreover, while the switching operation of themain switching device is being stopped in burst switching control,supply of the operating power for driving the main switching device isstopped. Thus, switching between burst switching and continuousswitching can be performed with high accuracy. Moreover, while theswitching operation of the main switching device is being stopped inburst switching control, the supply of the operating power for drivingthe main switching device is also stopped. This helps reduce the powerloss suffered while the switching operation is being stopped, and thushelps reduce the power consumption of the apparatus as a whole.

GLOSSARY OF TERMS AND ACRONYMS

-   PWM: Pulse-width Modulation-   IC: Integrated Circuit-   CS: Capacitor for Soft Starting-   FB: Feedback

1. A switching power supply apparatus having a serial circuit, includinga primary coil of a transformer and a main switching device, connectedbetween a positive and a negative power supply line connected to adirect-current power source, the switching power supply apparatusoutputting a direct-current voltage obtained by rectifying with arectifier a high-frequency voltage induced in a secondary coil of thetransformer by the main switching device performing switching operation,wherein the switching power supply apparatus uses as a feedback signal aresult of comparison between the direct-current voltage and apredetermined reference voltage, and drives the main switching device byturning on and off, according to a signal level of the feedback signal,supply of operating power to a main switching device driving system thatdrives the main switching device.
 2. A switching power supply apparatushaving a serial circuit, including a primary coil of a transformer and amain switching device, connected between a positive and a negative powersupply line connected to a direct-current power source, the switchingpower supply apparatus outputting a direct-current voltage obtained byrectifying with a rectifier a high-frequency voltage induced in asecondary coil of the transformer by the main switching deviceperforming switching operation, wherein the switching power supplyapparatus further includes: an output voltage detector that compares thedirect-current voltage obtained through rectification with apredetermined reference voltage and that outputs a result of thecomparison as a feedback signal; a switching controller that drives andcontrols the main switching device according to the feedback signaloutput from the output voltage detector; a signal level checker thatmonitors a signal level of the feedback signal and that outputs anoperation control signal for turning on and off the switching controlleraccording to the monitored signal level; and an operation/nonoperationswitcher that is provided in a line by way of which the switchingcontroller is supplied with operating power and that turns on and offthe switching controller according to the operation control signal fromthe signal level checker, the switching power supply apparatusoutputting a desired voltage by driving the main switching device with adrive signal from the switching controller that is so turned on and off.3. A switching power supply apparatus as claimed in claim 2, wherein thefeedback signal from the output voltage detector is transmitted to theswitching controller through a photodiode of a photocoupler, and thesignal level checker monitors the signal level of the feedback signal bycomparing a current level flowing through a phototransistor of thephotocoupler with a reference current level.
 4. A switching power supplyapparatus as claimed in claim 3, wherein a current detection resistor isconnected in series with the phototransistor of the photocoupler, andthe signal level checker turns on and off the switching controller byfeeding the switching controller with, as the operation control signal,a signal obtained by comparing a voltage drop across the currentdetection resistor with a voltage of a current level check referencepower source.
 5. A switching power supply apparatus as claimed in claim3, wherein operating power of the signal level checker and thephototransistor of the photocoupler is supplied from subsidiary controlpower extracted from a node between a plurality of diodes constituting aserial circuit provided in a steady-operation current supply line by wayof which a voltage induced in a subsidiary coil of the transformer issupplied after being rectified with the plurality of diodes.
 6. Aswitching power supply apparatus as claimed in claim 2, wherein theoperating power of the switching controller is supplied by way of astart-up current supply line by way of which a start-up current issupplied from the positive power supply line through a start-upresistor, or by way of a steady-operation current supply line by way ofwhich a voltage induced in a subsidiary coil of the transformer issupplied after being rectified with a serial circuit composed of aplurality of diodes, and operating power of the signal level checker issupplied from subsidiary control power extracted from a node between theplurality of diodes.
 7. A switching power supply apparatus as claimed inclaim 2, wherein the switching controller is realized as a pulse-widthmodulation (PWM) control circuit that outputs, as the drive signal withwhich to drive the main switching device, a pulse signal that ispulse-width-modulated according to a voltage level of the feedbacksignal from the output voltage detector.
 8. A switching power supplyapparatus as claimed in claim 7, wherein used as the PWM control circuitis a PWM control integrated circuit (IC) that is realized as anintegrated circuit chip having at least a feedback(FB) terminal to whicha voltage related to the feedback signal is input and a capacitor forsoft starting(CS) terminal to which a voltage for enabling or disablingan internal circuit is input.
 9. A switching power supply apparatus asclaimed in claim 2, wherein, when the pulse-width modulation (PWM)control integrated circuit (IC) is used as the switching controller, astart-up corrector is additionally provided to correct start-up of thePWM control IC; a first resistor is connected between a feedback (FB)terminal of the PWM control IC and the negative power supply line; thesignal level checker feeds a capacitor for soft starting (CS) terminalcontroller, which serves as the operation/nonoperation switcher, and theFB terminal with the operation control signal and an inverted feedbacksignal, respectively, according to a result of checking of the signallevel of the feedback signal; the CS terminal controller connects anddisconnects a CS terminal of the PWM control IC to and from the negativepower supply line according to the operation control signal; and thestart-up corrector connects and disconnects, through a second resistor,the FB terminal to and from the negative power supply line according toa voltage level of the subsidiary control power.
 10. A switching powersupply apparatus as claimed in claim 9, wherein the CS terminalcontroller includes an NPN-type transistor having a collector thereofconnected to the CS terminal of the PWM control IC, having an emitterthereof connected to the negative power supply line, and having a basethereof connected to the collector of the other of the transistorsincluded in the signal level checker.
 11. A switching power supplyapparatus as claimed in claim 9, wherein the start-up correctorincludes: a serial circuit composed of a Zener diode and a plurality ofresistors connected between a line of the subsidiary control power andthe negative power supply line; and an NPN-type transistor having a basethereof connected to a node between the resistors, having a collectorthereof connected through the second resistor to the FB terminal of thePWM control IC, and having an emitter thereof connected to the negativesupply power line.
 12. A switching power supply apparatus as claimed inclaim 9, wherein the signal level checker includes, for generation ofthe reference voltage, voltage division resistors, of which alower-potential-side resistor is divided into two resistors, with a nodetherebetween connected through a diode to the CS terminal of the PWMcontrol IC.
 13. A switching power supply apparatus as claimed in claim9, wherein the switching power supply apparatus further includes: acapacitor connected between the CS terminal of the PWM control IC andthe negative power supply line; and a diode connected between thecapacitor and the CS terminal.
 14. A switching power supply apparatus asclaimed in claim 2, wherein the signal level checker includes a pair oftransistors having emitters thereof connected together to form acomparator, with a base of one of the transistors connected to a nodebetween the current detection resistor and the phototransistor, with abase of the other of the transistors connected to the current levelcheck reference power source, with a collector of the one of thetransistors connected to a feedback (FB) terminal of a pulse-widthmodulation (PWM) control integrated circuit (IC), and with a collectorof the other of the transistors connected to a capacitor for softstarting (CS) terminal controller.
 15. A switching power supplyapparatus as claimed in claim 2, wherein, when a pulse-width modulation(PWM) control integrated circuit (IC) is used as the switchingcontroller, the switching power supply apparatus further includes: acurrent adjuster connected between a feedback (FB) terminal of the PWMcontrol IC and the negative power supply line to adjust a current outputfrom the FB terminal according to the signal level of the feedbacksignal; and a capacitor for soft starting (CS) terminal controller thatserves as the operation/nonoperation switcher by connecting anddisconnecting a CS terminal of the PWM control IC to and from thenegative power supply line according to an output signal of the signallevel checker.
 16. A switching power supply apparatus as claimed inclaim 15, wherein the current adjuster includes an NPN-type transistorhaving a collector thereof connected to the FB terminal of the PWMcontrol IC, having an emitter thereof connected through a resistor tothe negative power supply line, and having a base thereof connected to aline of the feedback signal.
 17. A switching power supply apparatus asclaimed in claim 15, wherein the current adjuster includes an NPN-typetransistor having a collector thereof connected to the FB terminal ofthe PWM control IC, having an emitter thereof connected through aresistor to the negative power supply line, and having a base thereofconnected to a line of the feedback signal, and in series with theresistor connected between the base of the NPN-type transistor and thenegative power supply line is connected an NPN-type transistor having acollector and a base thereof connected together.
 18. A switching powersupply apparatus as claimed in claim 2, wherein, when a pulse-widthmodulation (PWM) control integrated circuit (IC) is used as theswitching controller, a start-up corrector is additionally provided tocorrect start-up of the PWM control IC; a start-up switcher isadditionally provided to turn on and off supply of operating power tothe signal level checker; a first resistor is connected between afeedback (FB) terminal of the PWM control IC and the negative powersupply line; the signal level checker feeds a capacitor for softstarting (CS) terminal controller, which serves as theoperation/nonoperation switcher, and the FB terminal with the operationcontrol signal and an inverted feedback signal, respectively, accordingto a result of checking of the signal level of the feedback signal; theCS terminal controller connects and disconnects a CS terminal of the PWMcontrol IC to and from the negative power supply line according to theoperation control signal; the start-up corrector detects whether or notthe feedback signal is present so that, if the feedback signal ispresent, the start-up corrector connects, through a second resistor, theFB terminal of the PWM control IC to the negative power supply line and,if not, the start-up corrector cuts off the second resistor; and thestart-up switcher detects whether or not the feedback signal is presentso that, if the feedback signal is present, the start-up switcher turnson supply of the operating power to the signal level checker and, ifnot, the start-up switcher turns off supply of the operating power tothe signal level checker.
 19. A switching power supply apparatus asclaimed in claim 18, wherein the start-up switcher includes an NPN-typetransistor having a collector thereof connected to a node between acurrent detection resistor connected to a line of the feed back signaland an internal reference voltage line of the signal level checker,having a base thereof connected to the phototransistor, and having anemitter thereof connected to the negative power supply line.
 20. Aswitching power supply apparatus as claimed in claim 18, wherein thestart-up corrector includes an NPN-type transistor having a collectorthereof connected through the second resistor to the FB terminal of thePWM control IC, having a base thereof connected through a resistor tothe phototransistor, and having an emitter thereof connected to thenegative power supply line.
 21. A switching power supply apparatus asclaimed in claim 2, wherein, when a pulse-width modulation (PWM) controlintegrated circuit (IC) is used as the switching controller, a start-upcorrector is additionally provided to correct start-up of the PWMcontrol IC; a first resistor is connected between a feedback (FB)terminal of the PWM control IC and the negative power supply line; thesignal level checker feeds a capacitor for soft starting (CS) terminalcontroller, which serves as the operation/nonoperation switcher, and theFB terminal with the operation control signal and an inverted feedbacksignal, respectively, according to a result of checking of the signallevel of the feedback signal; the CS terminal controller connects anddisconnects a CS terminal of the PWM control IC to and from the negativepower supply line according to the operation control signal; and thestart-up corrector detects whether or not the feedback signal is presentso that, if the feedback signal is present, the start-up correctorconnects, through a diode and the second resistor, the FB terminal ofthe PWM control IC to the negative power source line and turns on supplyof operating power to the signal level checker and, if not, the start-upcorrector cuts off the diode and the second resistor and turns offsupply of the operating power to the signal level checker.
 22. Aswitching power supply apparatus as claimed in claim 21, wherein thestart-up corrector includes an NPN-type transistor having a collectorthereof connected through the diode and the second resistor to the FBterminal of the PWM control IC, having a base thereof connected througha resistor to the phototransistor, and having an emitter thereofconnected to the negative power supply line.
 23. A switching powersupply apparatus as claimed in claim 21, wherein the signal levelchecker includes, for generation of the reference voltage, voltagedivision resistors, of which a lower-potential-side resistor is dividedinto two resistors, with a node therebetween connected through a diodeto the CS terminal controller, and the CS terminal controller isconnected through another diode to the CS terminal of the PWM controlIC.
 24. A switching power supply apparatus as claimed in claim 2,wherein, when a pulse-width modulation (PWM) control integrated circuit(IC) is used as the switching controller, a start-up switcher isadditionally provided to turn on and off supply of operating power tothe signal level checker; a current adjuster is additionally providedthat is connected between a feedback (FB) terminal of the PWM control ICand the negative power supply line to adjust a current output from theFB terminal according to the signal level of the feedback signal; thesignal level checker feeds a capacitor for soft starting (CS) terminalcontroller, which serves as the operation/nonoperation switcher, withthe operation control signal according to a result of checking of thesignal level of the feedback signal; the CS terminal controller connectsand disconnects a CS terminal of the PWM control IC to and from thenegative power supply line according to the operation control signal;and the start-up switcher detects whether or not the feedback signal ispresent so that, if the feedback signal is present, the start-upswitcher turns on supply of operating power to the signal level checkerand, if not, the start-up switcher turns off supply of operating powerto the signal level checker.
 25. A switching power supply apparatushaving a serial circuit, including a primary coil of a transformer and amain switching device, connected between a positive and a negative powersupply line connected to a direct-current power source, the switchingpower supply apparatus outputting a desired direct-current voltage bycontrolling the main switching device according to a feedback signalobtained as a result of comparison between a direct-current voltageobtained through rectification of a high-frequency voltage induced in asecondary coil of the transformer by the main switching deviceperforming switching operation and a previously set reference voltage,wherein a signal level of the feedback signal is compared with a signallevel of a previously generated oscillation signal; according to aresult of the comparison, an on-state duty of a drive signal to be fedto the main switching device is determined and switching between burstswitching control and continuous switching control is performed; andwhile switching operation of the main switching device is being stoppedin burst switching control, supply of operating power for driving themain switching device is stopped.
 26. A switching power supply apparatusas claimed in claim 25, wherein burst switching control is achieved byturning on and off supply of operating power to a switching controllerthat drives the main switching device.
 27. A switching power supplyapparatus as claimed in claim 25, wherein, when a pulse-width modulation(PWM) control integrated circuit (IC) is used as the switchingcontroller, a capacitor is connected between a feedback (FB) terminal ofthe PWM control IC and an internal power terminal connected to aninternal power supply line.
 28. A switching power supply apparatus asclaimed in claim 25, wherein, when a pulse-width modulation (PWM)control integrated circuit (IC) is used as the switching controller, aserial circuit composed of a capacitor and a resistor is connectedbetween a feedback (FB) terminal of the PWM control IC and an internalpower terminal connected to an internal power supply line.